Players are reluctant to raise with three cards, (overly) fearful of landing partner in a 4-3 fit. Here's why you shouldn't worry: When partner opens 1 ♥/♠ with just four cards, they have 15+ points (with 12-14, they'd normally have opened lNT). If they have 17-19 with four cards, they have enough to bid on. Only when they have 15-16 will you end up in the 4-3 fit 2 ♥/♠. And that contract will often play better than lNT. 

I recommend always making a three-card raise 1 ♥/♠ to 2 ♥/♠ (when the alternative is 1 NT) unless you have three small cards in a very flat hand.

Raise 1 ♠ to 2 ♠ with the first three of these, only bid lNT with the last:

The corollary to the frequent three-card raise is that opener must be wary after l♥/♠-2 ♥/♠ when he has a game-going hand with just four cards in the major. He cannot simply jump to 4 ♥/♠ (or invite with 3 ♥/♠) unless he has five+ cards. He must suggest notrumps, bidding 2 NT with 17-18 and 3 NT with 19 (passing 2 ♥/♠ with 15-16).