Answer
Against three notrump, partner leads the deuce of diamonds (five, Queen, King). Now the three of spades (deuce, Jack, nine). Now the Jack of hearts.
Should you cover? The auction suggests that declarer has exactly five hearts, so partner probably has two. Covering gains if partner holds 10x, makes no difference if partner has Ax or xx, and gives declarer a losing option if partner has Qx. You should cover.
Bonus from Val: A creative declarer might have only four hearts (covering is best) or six (partner might have singleton Ace or singleton Queen, so covering is not best), but both possibilities are unlikely. You should cover.
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