You can use TreeShare for Ancestry to keep your Ancestry tree current. You can use that same database to make changes on FamilySearch Family Tree. MyHeritage does not have an API that allows you to make changes to their website through RootsMagic. So MyHeritage will be the harder one to maintain. I would start with Ancestry, FamilySearch and then upload new GEDCOMs to MyHeritage to keep that current.
I know about using TreeShare for Ancestry. What is the fastest way to make changes to my FamilySearch family tree using RM7?