It completely depends on what you want to do at Epcot, and how long you're staying at HS, and when you are getting to Epcot.
If you absolutely must do Soarin (or Test Track), then you want to save your FP+ for Epcot. (But note that Test Track is on the same FP "tier" as Soarin so you can't reserve FP+ for both). Rope drop at HS on a crowd level 3 you should definitely be able to get in multiple rides on both RnRC and ToT the first hour the park is open. My best morning was 5 RnRC and 3 ToT in about 2 hours (although some were with old paper FP).
But if you are just going to Epcot for Food & Wine or dinner/fireworks and don't care so much about rides, book the 3 FP+ at HS. Even on a "3" day, at a small park like HS, the headliners could easily have lines over 30 minutes an hour after opening, especially if its a weekend. With FP+, you can get a couple more quick rides midday when crowds pick up. And you can get more FP after the first 3 are used.