Hi my daughter is going to disney Sept 2020 with her husband. They want to do islands od adventure and universal over 3 days. What is the cheapest way to do this ?

If you're a Florida resident the are resident discounts. Best bet is to try that if you qualify, otherwise a multi park multiday ticket with fast pass is going to be the least expensive ticket.

Try AAA and keep asking around. There may be some web sites that offer more reasonable options

I looked at a number of tickets providers to see who offered the best deals, she'll need to do her research