According to a new USCIS memo, only last tax year's tax return is needed.
If you give me second, I'll find you the link to that memo.
I once called that service number and they told me that it would still be better to send tax returns for the last three years. But apparently a lot of people here only sent last year's tax return and it was fine. We only sent the last two.
So I'm positive that you will only need the tax return for the last tax year.