Can I sync all of my trees in one place from Ancestry, Family Search, and My Heritage? If so, how?
No. Each of them uses different ways of connecting to RootsMagic.
You can share data back and forth with Ancestry TreeShare and FamilySearch. Both options are on the Publish page. With MyHeritage you can receive hints but it won’t update your tree there.
as others said - yes with first TWO.
My Heritage would need GEDCOM – but they also make a desktop program.
(the is restricted by MyH not RM)
Kevin