Why do people get beat up when fighting 3 or more people at the same time when they claim they can box/fight?

The long answer by u_bin is pretty solid. I would only underscore one thing. Someone mentioning their skills usually means they are lying or bragging, which are not qualities of highly trained fighters. “The one who talks the most, usually knows the least.”

Apparently they overestimated their experience. 

it's usually the result of a combination of problems...

first, many people have a fighting mentality and train in martial arts for the wrong reason.... 

second, these people tend to become more socially overconfident the more skilled they become in a controlled, training environment...

third, they do not understand the difference between fighting as a sport and actual combat (which is what a street or bar fight actually is)..

put this all together and you have people who are prone to act aggressively when they should be practicing avoidance ... engage in a combat situation without properly assessing the threat.... and finding out too late that rules and technique goes out the window when an opponent's buddies decide to join in the fight ...

I think it was Chuck Norris who said that any credible martial arts and self defense program should begin with an emphasis on fight avoidance...covering everything from awareness (not getting into bad situations) to having the discipline to run whenever possible.