Brettanomyces will not sour a beer, it adds the barnyard funk.
Lactobacillus and Pediococcaus(sp) is what creates the sour twang that it seems your are looking for.
This is a good primer. http://www.themadfermentationist.com...r-at-home.html
IMO you should brew a beer with the intention of souring it, and plan the recipe accordingly. There is some debate as to whether you add the funk with your base yeast strain or after fermentation is complete.
90 days is not long enough for a beer with Brett or pedio. My advice to you would be do some research, and if you want to get a sour pipeline going, plan ahead from recipe formulation and expect to invest a year or two of waiting. In the meantime be happ you can buy jolly pumpkins delicious products while you wait.