Steve's answer raises a bunch of good points which already cover a lot what I would have to say on martial arts as well, so I would like to have this answer to be understood as complementary.
About multiple opponents in martial arts
Generally, martial arts rarely train multi-opponent situations. Aikido does so on higher levels, but as already said, this is rather about tai-sabaki for evasion and strategic positioning rather than actual engagement.
There are a few systems which to train multi-opponent situations with full contact under pressure.
Some jiu jitsu schools do have this as "randori" form with multiple attackers so that you never know from where and how (including weapons) you are attacked, even as part of higher belt gradings. That is great for training attention, positioning, and doing techniques under stress without prior notice. But as far as I have encountered that kind of test, people never attack coordinated but still one after another there. So quite unrealistic.
A bit higher up the scale of realism, Krav Maga (good schools, not McDojo style) adds constant stress and attacks with pads, kicks and fists from all sides (if you let them) plus above-described attacks with grappling and weapons. This makes you realise that you really cannot win, only hope to survive long enough. If you make it three minutes, you're a real badass.
The same probably applies to all viable self-defence systems.
How to train
Generally, if you want to be able to handle these situations, you have to train them. Full contact, without body armor (maybe head), under stress, no holds barred. When adding weapons, cushioned bats, shock knives and soft air guns should be used - no serious injuries, but you will feel mistakes nevertheless. If you ever did that, you realise that there is no magical key to win. It is a lot about attentiveness, movement, positioning, and distance. Sometimes, you can grab someone and use them as a shield. You can use any object as an obstacle. But it wears you out fast physically and mentally, no matter how well-trained you are.
The psychology of street fights
As often in street fights, psychology is a major factor: Finish the first encounters fast and brutally, don't let them see any pain or doubt, focus on those who seem to be group-leaders, and maybe the rest will run rather that get their ass kicked as well, or at least hesitate long enough to open up an opportunity for a run or finishing them off.
Assessing the amount of intent is very important in order to choose appropriate means and, maybe more importantly, targets. Street fights are all about a display of power, so one should use the least severe means necessary to unambiguously make clear that you are more powerful than they are. This begins with body language. Calmness, relaxation, focus, absolute sovereignty is what one should show.
Basically, if the situation is really threatening your physical health, you have to incite fear in the group, so once the fight begins - which you should only do if you know it is inevitable - tearing (ears and piercings bleed like hell if torn out), biting, screaming, scratching...if they believe you are a devil/dragon, most aggressors keep distance. The particular technique does not matter here. If you do a elbow, grab/ear-rip, knee, elbow combo and finish the boss in two seconds, fine. If you throw in one fluid attack and break their hip (and maybe arm for good measure) so that they scream in pain, great either. But you have to be fast and mobile either way, so do never engage for more than, say, two seconds. Otherwise, you will be cornered and attacked from behind.
Still, if you have two or more equally trained opponents between them who coordinate and attack with the intention to hurt you, there is no way you can do anything except running. Playing safe is no shame. And there is only so much you can do if they have real intent to hurt you personally as opposed to like to be violent and hit random people.
Another important psychological aspect is that even if you are the superior fighter and could more or less easily dispatch them in hand-to-hand, do never corner opponents to the point were they feel they cannot escape! The reason is twofold: Firstly, you will activate the full hormonal reaction, which gives them far higher strength and pain tolerance than normal. Secondly, even if they would not do so in normal circumstances, if they get the chance, they will use lethal force out of sheer fear (weapons, mostly). Thus, you make them a more serious threat without need. Thing is, in many countries this would legally count as self-defence in their favour since you used excessive violence and went beyond reasonable force.
Real world example: There has been an encounter here in Berlin, Germany where a full-contact karate competitor was attacked by a group of ten. He dispatched five fast and brutally. The rest ran. He ran behind and cornered one guy. This guy only then pulled a gun and shot him lethally. Even though the gun and carrying it was illegal, it was judged as appropriate self-defence (rightfully so, my two years of studying law tell me), so the guy was only sentenced for illegal gun possession and attempted robbery.
On using weapons
Having a jo makes things considerably easier if you know how to use it, bats or knives don't help much. But you don't walk around with a 6ft staff all the time. And as soon as you are circled and they are trained and want to get you, you're still done. Generally, the above points about psychology apply here as well: If you use weapons, you both seem less of a deamon (more insecure, normal) and make the use of lethal force on their side more probable. Looking very cold-blooded with a gun in your hand counting people (by aiming) you can get down before having to reload (given that is allowed in your place) does help to intimidate people as well. A good, but again not exactly viable (due to it being forbidden to carry it in most places) choice of weapon would be a sharp sword, since it is good for distance, fast enough, and intimidating/dangerous.
Weapons are the point where the difference between martial arts and self-defence becomes most obvious, IMHO. Most of the martial arts weapons training is either useless because you are not carrying these weapons (it is WAR - thus martial arts - where you always carry weapons proper, not civil life), or because it is a step of escalation one would rarely take, or the techniques are actually only effective in a specific ruleset or situation, eg. a single opponent with the same weapon (think Kali).
Hit fast, hard, and brutally. Target those most intent/powerful first in order to break down their morale and chain of power. Keep moving. Use obstacles (people can be obstacles as well, especially frightened offenders). Do not corner people. Don't let yourself be cornered. Do not use weapons (unless your local law allows to shoot all attackers before they can do anything and you know you can pull that feat off maybe). If you get the chance, always choose to get the hell out of there before anything else. One never knows what will happen and a single mistake/hit may end your health or your life. Only life itself is worth risking one's life in the streets, no property, "honour", or pride is.