mating site must be a3-square. white job is to stop black king to reach c-file. since black pawn can’t move, white can only use zugzwang. so 1. Qh6 (to reach c1 to banish black king to c-file and escape). 1. … Kb4 (forced in zugzwang; pawn move allows mate in a6). Then 2. Qc1 (to protect c-file and c-pawn from king’s escape). Then 2. … Ka4 (or a5) and 3. Qa3+ checkmate.