Но он сказал, что мы должны сделать

Но он сказал, что мы должны сделать все упражнения в письменной форме.

Results (English) 1:

But he said that we have to do all the exercises in written form.

Results (English) 2:

But he said that we should do all the exercises in writing.

Results (English) 3:

But he said that we should do all the drills in written form.