Not specifically the type of person you're looking for an answer from, but I'm a first-year JET still considering to leave after the 2016-17 contract expires.

However since getting here I've learned that most jets stay 2-3 years (...)

Yeah this sounds about right. The guaranteed pay raises until year 4 are a huge incentive for people.

(...) and most people in my office are under the impression my contact IS for 3 and that's how long I'll be staying for starters.

I didn't read your contract, but I assume your contract is only for 1 year, right? Then your coworkers are just assuming things that were never said.

You're under no obligation to recontract longer than you'd like. A lot of people have been asking me how long I plan to stay, also assuming I'll stay 3-4 years (I don't know where they got this specific idea), and when they ask me I tend to say "well, it depends on homesickness, it depends on the US economy, it depends on adjustment to Japanese culture, so I'll like to stay a long time but I can't know yet!" and I get a lot of 「おー、そうですよね、なるほどですね」

So maybe I'm just being young, foolish, and brash, but if I choose not to recontract for next year and someone really tries to convince me otherwise, that's their problem, not mine: I'm under no obligation for anything outside the contract I already signed, and if I think my future is better moving back home then I'm not going to make myself suffer doing something I don't want to do.