Hi,
I thought I'd start a new thread so that those trying to upgrade to Yosemite might also see a new thread that it worked well for at least some of us. Perhaps this information will be diagnostic for others.
I waited to upgrade to Yosemite until I read that FTMM3 was compatible with Apple's new operating system. I then upgraded to Yosemite without incident and was able to use my FTMM3 program. Today, I noticed that there was an update to FTMM3 to fix bugs with Yosemite, so I prepared the copy of my FTMM3 file as directed in the update notice from FTMM and downloaded the update for FTMM3 (version 22.2.3.761). I have now used FTMM3 without incident and can sync my tree. With fingers crossed, all seems to be working fine.