I have a 2010 Mazda3 5 door and after 3 months the rear hatch door won't stay open. Anyone else had this problem?

Arnie

 

You should take back the hatch door and get a new car.

It's probably just a bad riser.

 

+1 either one or both of your riser struts lost pressure and can't hold the weight. It should be easy to pop each off and feel the tension against them and see which (or both) are dead. Regardless it's under warranty so just take it back and get them to fix it under warranty.