You should bid 1♦. Bidding a new suit forces Partner to bid again. Although only having 3 cards in the suit you have strength in ♦s. The experts call this "bidding a false suit".

Your 1♦ bid is a positive step towards finding a NT contract and in the rare case that Partner raises your ♦s (in which case he holds 5 ♣s and 4 ♦s), you can easily raise Partner's ♣s.

In the deal at hand Opener replied with 1NT, showing a balanced hand of 13-15 points without a 4-card major.
East's turn again : what should he do ?

PS
When you are forced to bid a false suit, pick one Partner is unlikely to support. This usually is a minor suit, but it can, on rare occasions, also be a major suit of which Partner has already denied having 4 cards in.