It makes sense to have the option to do 3v3's in TMM. People can queue for multiple things simultaneously.
Numerous players do like to play 3v3's. Also, sometimes groups of 3 friends want to queue together for 3v3's where they are the entire team (without a random 4th teammate with them).
If you need help finding good 3v3 maps, I can help with that if you'd like, and there is also the option of having the 3v3 queue mostly or entirely use map gen maps, if desired.
PS: Having multiple 4v4 TMM options does not have to be mutually exclusive with having 3v3 TMM available.