To get a sign-off (a recommendation that you are proficient and prepared) from any instructor, you have to show to him that you are.
If you can sit for a 2-hour oral, and fly a 1.5 flight doing all the PTS maneuvers well within standards all the time convincing the new instructor that you are always in control of yourself and your airplane and you never have to be guided, doing it just as you would do a checkride, then he does not have to actually fly 3 hours with you.
The regs say 3 hours within the last 60 days in preparation, but not necessarily with the same instructor.
But...what I said above is not going to happen. Not unless you happen to know another instructor in the same school who knows you, and has sorta followed your progress and can honestly evaluate you in 1 session like that and you perform almost flawlessly. And still, that is putting the CFI in a corner. We all like at least 2 consecutive good performances.
So, the 3 hour rule with the same instructor is kinda necessary.