There’s a one-word key to finding the answer: subjunctive. That’s the term for the grammar dynamic that determines whether “was” or “were” is best here. Armed with that one little word, you can ...
“If there were a Form 3, you would have already filled it out.” Reader Jessica had a question about a sentence like this. The speaker already knew about a Form 1 and a Form 2. The existence of Form 3, ...