For the 3rd level, select the squares A, B, C, D, E, B, E, D, C, F, G, F, C, D, E, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, and V.
This solution works for both difficulties.
You earn the TRAINED MOUSE.
Walk down twice.
Place the TRAINED MOUSE in the hole (A).
Take the KNIGHT (B).
Go up the stairs.
Place the DRAGON and the KNIGHT on the chest (C).
Take the 3/3 FEATHER (D).
Walk down and right.