I've seen too many situations where someone has relocated only to find out the company really doesn't have a job for them anymore.
Worst was when MIT executed the Applied Biology department in the middle of the night, with one new junior faculty member who'd turned down other good offers showing up right then, family and all as I recall. I also had a roommate who was going to start as a grad student that fall....
I cite the above because the people hiring you might not have the power to keep their word. Depending on the market, internal politics, etc., they might not even have jobs 3 months from now. The Applied Biology faculty and staff made those offers in total good faith, they just lost their ability to make good on them (because they were too good at getting research money, but that's another story).
Also note that that job offer is highly subject to going "poof" when there's that long a delay to your first day of work. Certainly don't give notice before you're seriously committed (e.g. bought a new house "over there").