What kind of Sedna build were you playing? And what maps are you planning on playing vs. PCG? You can have a lot of fun pushing speed with Sedna, running in and pouncing and running out, letting monks heal you, then running in again. That having been said, I've never had much luck staying in a game that goes long with a Sedna, but I've seen people do it.
I highly recommend getting experience at a few guys, then pick your characters based on what they pick. For example, if they play minions, UB's ooze can be the MVP.
What's your favor item of choice? Getting the teleportation amulet as, well, anything in 3v3 lets you jump sides, get exp you'd miss out on otherwise, and really screw with your opponent.
ADDITION EDIT:
On the strategy front - I think what makes Demigod so amazing is the number of strategic decisions you make per minute. What flag do you focus on? He's coming at you - run, stay, cap then run? Have a team mate come help? Should I upgrade items or creeps? Looks like Rook's coming over to help - should I chase him any further? I've played a lot of strategy games where there are really cool options to make your player/team unique, but the number of pressured strategic choices are limited.