I would get three yards - a few inches extra to allow for shrinkage (if you wash your fabric before using it)
But then, I'm a believer in "It's better to have it and not need it, than to need it and not have it"
90 inches is 2.5 yards - and I would allow an extra 1/2 yard to allow for shrinkage and for the long arm person (if you do the LA route)