The difference in both health and damage per hit.
You can see the full comparisons on a Wiki-page here. (Data may be outdated, if the balance has been or will be tweaked, but I doubt it's going to be a big difference).
As there are 6 different team veriations, 1v1, 1v2, 1v3, 2v2, 2v3 and 3v3, there are a lot of different numbers to thorw around. But, in uneven situations, the boosts are surprisingly consistant.
I'm comparing these numbers to a simple 1v1 health and damage values.
- In 1v2 and 1v3, the one always gets boost of 105% (so, over doubling) in health and 45% in damage
- In 2v1 and 2v3, the two always gets boost of 15% in health (per character, total boost of 130%) and 30% in damage
- In 3v1 and 3v2, the three never gets any boosts (per character, total healt is boost of 200%)
- 1v1 and 2v2 are fought with normal numbers. In 3v3 both teams get a boost of 15% in health (per character) and 30% in damage
Teams of two or three also get the added benefit of extra moves, like Ensambles, Blockbuster Sequels and Stunt Doubles, which solo characters can't use, as well as slight health regen (to a point) while Tagged Out.