Oops, I've forgotten about -щ! It is the same case as that of -ч:
The phoneme is always soft, but the combinations щя, щю, щё should be spelled as ща, щу, що.

Плащ, плаща, плащу, плащом, плаще, плащи, плащей, плащам, плащами, плащах.

-щ, -ща, -щу, -щом/-щем, -ще, -щи, -щей, -щам, -щами, -щах