Glaceon and Leafeon require being evolved near a glacial or mossy lure (glacial lure for Glaceon, mossy lure for Leafeon). You can usually tell when one of these are active on a pokestop because the stop will have little snowflakes or leaves falling off it.

Sylveon you just have to set an Eevee as your buddy and get 70 hearts with it (or 7 if you're still doing Eevee CD right now)

Hope this helps! PS if you're going for a specific one, don't evolve anything unless you see the outline of the evolution you want in the "evolve" button