BO3 = Best of three, meaning you have to win 2 maps in order to win the match.

You start with pistol bash.
Loser picks his map first.
Winner picks the second map.


If both of you won a map you will have to veto the remaining maps, this means both teams remove one map and the remaining map is the 3rd map.

Just like in any other ClanBase match, you can pick sides on the opponents map.