Does Eb3 (employment based) visa require us to submit copies of previous year's tax return during any part of the application process?

I'm sure you understand why I am asking this, and don't want to put myself at risk of rejection due to not being able to submit tax returns.

Don't think so.
But just curious, why would it be a problem.

yo barsa f1 ma social use garera bahira kaam gareko chu

By law, you do not need it, assume you did not even file it.
However, if you do get requested for it you still fine because tax docs will not have info about how long you worked just how much you made. For all it matter is could have been an oncampus work as well which does not require work permit.
Unauthorised work is considered permissible or I would say ignored if its less than 180 days for 485 application.

I say you are fine. Don't submit things that you dont have to.

Fdpower, thanks for the info. I completely missed the fact that tax returns don't have employer name. I think I should be good then.

मेरो केस मा पठाएको थियो ! बत मैले opt मा काम गरिरहेको थिई ! अन्य इल्लेगल काम गर्य छ भने नपठाएको नै राम्रो !!!

I didn't submit my Tax return while filing for the I-485. I don't think that is required for the application.