If [PMType] truely is a text field (as implied by your original post), then what Ken said...
=IIf(([PMtype]="3") Or ([PMtype]="4") Or ([PMtype]="5"),2,1)
If it is actually a numeric field type, you would need to omit the quotes...
=IIf(([PMtype]=3) Or ([PMtype]=4) Or ([PMtype]=5),2,1)