Если бы не серьезное выражение его

Если бы не серьезное выражение его лица, мы приняли бы это за шутку.

Results (English) 1:

If not for the serious expression on his face, we would have taken it for a joke.

Results (English) 2:

If it was not a serious expression on his face, we would take it as a joke.

Results (English) 3:

If it were not a serious expression of its person, we would have taken it for the joke.