I think PC-Task 3.1 is the same as 4.4 (that I'm using)....
In that, you have to configure all the screenmodes from the start-up options, otherwise it'll use defaults, i.e. native screen modes.
You'd usually want to use 640x480x8 I suppose for Windows 3.1, maybe 800x600x8 - check particularly screenmodes like that are set properly.
Incidentally, you can't run Win '95 on less than version 4 ofPC-Task, and even then it's not really usable.