Not how it works. Best of 3 is first to 2. Best of five is first to three. Best of seven is first to 5. Best of 10 is first to 7.
Source:
I've been in lots of tournies/games that follow such formats across different genres both in real world and online games.
Best of seven, first to 5 doesn't even make sense.
First team wins 4, second team wins 3. It's already out of 7.
NBA playoffs are best of 7, first to 4.
Or maybe gamers just follow some illogical rule. Beats me. Lets have fun.