3 boxing you basically are limited to doing 3's yourself, or playing with friends.

You can be successful in 3's by yourself but it takes a lot of time and effort to get everything setup correctly. I disagree with ughmaheadhurts it is more complex to do REALLY well with opposed to 4-5 boxing 5's.

Ultimately it just comes down to if you want social interaction or not. I do that with 4 toons though because I either dual box or single box 3's on all my toons as well as doing 5's as a 4 box group.