Bro, best of 5 and first to 3 is the same thing.
I always mention both cause I thought it was helpful to understand.
It's not hard to get it, listen: The first match you will play is gonna be on abd, the second in lenshu, the third in erthtk, the fourth in rkmma and the last match in boxshu. What happen with that, if u reach 3 wins without losing any, rkmma and boxshu won't be played. Then if you lose 1 fight but you win 3 (total of 4 fights) you are gonna play in rkmma but u won't play in boxshu. AND the last possibility is that you lose 2 fights but you win 3 (total of 5 fights) and in that case you will use all the mods.
Is it clear now? (it's actually really simple)