Ultimate Weapons

Hi there.

Is there a way to reroll which ultimate weapons you can choose when get enough triangles? Took me about 2 months to get 800 and a UW I want isn't there. Not sure why I can't pick from a list, only 3 but is there a way to change which 3 I have the option of, or do I need to wait 3 more months before I get another shot?