Infinity War and its sequel (unnamed, they've said it won't be called Infinity War part 2) are just the last movies of phase 3, just as the previous Avengers movies were the last of phase 1 and 2 (well, besides Ant-Man, which got switcharoo'd to the end of phase 2). Some of the current avengers' contracts will be over at the end of phase 3 but whether they'll kill them off, retire them or just re-up the contract, who knows.

Obviously they'll just go right ahead with phase 4, and probably phase 5 too. Expect more space and magic at that point, phase 1 and 2 started off relatively grounded so as to better invite newcomers to crazy comics shenanigans.