[Q] Is there a regulation that says I-829 should be decided within 3 months from the filing date?
Yes, below is the language. However, the below regulation is not being followed.
(1) The decision on the petition [I-829] shall be made within 90 days of the date of filing or within 90 days of the interview, whichever is later.
Ironically, if you do not file I-829 within the 90-days window period, then your CPR will terminate, but if USCIS does not adjudicate I-829 within 90 days as stated above, nothing happens.
The above regulatory language clearly shows that the Congress (and actually, USCIS itself) at one time thought it was perfectly reasonable or necessary that I-829 be decided within 90 days from the filing date.