Hi, there. The 90 day limit is based on your insurance. If your doctor writes a script for say 90 days, with 3 refills, you can get it all 360 days at once, but your insurance won't pay for it. (There are exceptions to this like opiates, and it may vary by state.)
In 2016, I was taking a drug that I could get at Costco, 90 day supply for $16, cash pay. My insurance required pre-auth and limited it to 30 days with a $30 copay ($80 total cost at Walgreens). It was Costco that told me I could get the whole 360 if I wanted to. It was my doctor that told me to go to Costco. You don't have to have a membership to get prescriptions at Costco.
Most fill limits are cost reduction strategies set by the insurance companies.
(I did this before I had a HDHP. When you have that, you also have to weigh not having the expense included in your OOP max. I hate our system.)