You should have a back-up of every tree file, a back-up is automatically created, these files have "auto-backup" in the title.
If the original is corrupted, then the auto-backup of it should be used. Do you use any device to back-up your machine data? That's also another source for an uncorrupted file.
Do you know if the tree file created on FTMM(2010) was ok before you attempted to open it in FTM for Mac3?
Do you have a corresponding file (same name tree) on ACOM?
Here's what I would attempt first.
1- If FTM for Mac3 is signed-on to your ACOM account, sign-off and quit the program.
2- If FTMM(2010) is signed-on to your ACOM account, sign-off and quit the program.
3- Perform a permissions repair on you machine. I don't know what OS your machine is using, but it's pretty simple and quick, the directions to do this are on the Apple site, search "disk utility" in whatever you OS is.
4- Once your machine has rebooted from the permissions repair, open FTM for Mac3.
-Pull down the FILE (next to Family Tree Maker 3, the very top of your screen) menu, select OPEN. All the tree files and back-ups created by (any version) FTM should be in a list. Select the file you want to open, I would first try the auto-backup file of the tree you were having problems with.
If that doesn't work, come back and we'll try something else.