Honestly I don't think its the gamemode, but more that the level design for 3CP is approached in the same way as 5CP, with a hard to push last.
I think that it was really noticeable in cp_tidewater that the final point was fairly easy to cap, even if only a few people assaulted it. This kept the map from stalemating, but there was still a lot of to and fro across mid.
TL: DR Tidewater's mid is sort of stalematey and it's last is easy to attack, and it made 3CP really fun