>>93429477First 4 full episodes of Season 3 will be premiering all at once July 15th as a "TV movie" called The Battle for Mewni. Directly continues from the last finale, seems to involve a flashback to Moon's history with Toffee as well. There are some promos online already.
The new OP animation came out recently as well. After the premiere the rest of the season should begin airing in September. It and the 4th are supposed to be 21 episodes each.
That's about it, assuming you watched all the new episodes in February.