Seddie, Creddie, Cam.
The third is unlikely to happen. No, all three are unlikely. And I'm about to give you a rant explaining why because I can (thought you might want to just wait 2 weeks for my guesses to be proven wrong, this will get somewhat bleak).
If Sam ends up with Freddie; then something goes HORRIBLY wrong between them that Sam is now living with Cat in Sam & Cat. Carly no longer exists in the Sam & Cat continuity, because like Nathan, Miranda is not staying with Nick. The best hope is for Miranda to cameo in Sam & Cat (since Schnieder likes to reuse actors).
If Carly ends up with Freddie; then they get written out of the established universe (sad for us), and that leaves Sam on her lonesome, her two best friends in the world (and two-fourths of the people who acknowledge her value as a person). However, this does clear up the romantic triangle well and gives Sam room to move into her own universe/spin-off. But what a downer ending.
If Carly ends up with Sam; then something goes horribly wrong and parents flip-out over a lesbian relationship on a child-friendly show. Sam & Cat gets canceled, and Dan Schnieder will most likely never produce a show again. That's bad. Very bad.
So the best route? Nobody gets together. Look at the promotional material.
- We see Carly inside the elevator and say "I love you guys" in a voice that makes you want to cry. No Creddie or Seddie in that scene at all. CREDDIEAM, but not Cred or Sed.
- iGoodbye is about Carly getting her dad to take her to a dance. But we all saw the picture of Carly on a plane, not kissing Freddie. Clearly, she loses out.
- Sam & Cat exists. Freddie isn't a supporting character. Clearly, Sam loses out.
It ends neutral. No Creddie/Seddie/Cam (awwww...)
Now, the ending is what has me puzzled. Who actually leaves?
- Carly's on a plane in the Twitter pic. But why is she leaving? The plot's about her contacting her dad again. Why does she end up moving, or taking a plane to go and visit/live with him?
- Sam's moving into a place with Cat. Cat goes/went to Hollywood Arts - Hollywood - that's pretty far from Seattle.
- Freddie probably got a scholarship somewhere. Sam ruined his chances last time. But they haven't graduated yet (apparently) if they're going to a school dance.
Maybe Carly stays while Sam and Freddie leave? Maybe Freddie stays (we see him in the hallway getting his camera equipment) while Sam and Carly leave?