I've been on a 10 day cruise and that was too much, can't imagine 3 years. Even if they let you opt out after a few months...nah. I had a blast but the longer you're on a cruise, the more you start seeing things that make you realize you gotta get out of there lol. You start hearing about people getting sick and don't think much about it, then you realize damn such and such been in their room for days. Then you start hearing things. Every day on the cruise I remember hearing an old person (who I didn't know) saying something like "did you hear Bill hasn't come out his room today? He didn't sleep last night and is too tired." Even as a kid I felt more tired as the days went by. You just feel cramped up, even if you walk around the ship.
Your entire perception of time changes. The ship never truly shuts off or goes to sleep, and many a night I'd find myself walking around eating an ice cream cone and then realizing oh shyt it's 3AM fukk. Go back to your room, try to go to sleep and suddenly it's 8AM and you slept maybe an hour. Time to get up and go to that nice breakfast spot you found lol. It feels like being in Vegas too long, but way worse since you aren't on land.