Underlayment for low slope roofs

Should I use peel and stick for a roof with slope 3/12?

I would suggest it over standard underlayment.

That or 2-layers of underlayment are required in our area if below a 4/12.

It depends on what type of roof you are installing. IMO anything less than a 4/12 should not be shingled no matter what type of underlayment you use.

Depends where you are located. If you are in the snow country, I’d agree about not using comp. If you are in the west, you can get by for nothing lower than 3:12 (if you don’t read code requirements) and should use a wind and water shield type material under the shingles. At that slope the shingles are just for looks and to protect the underlayment. Code does allow two layers of underlayment in lieu of the peel and stick membrane but does ask for mastic between layers (which no one does).