are you machine quilting your sandwich, sending it out to a long-arm(er), hand quilting, tying? it all makes a difference. your backing (for machine quilting) should be 4-5" larger all the way around than the quilt top (ie: if your top is 72" x 90" your backing and batting should be at least 82"x 100", then after the quilting is done it is trimmed when you square up your quilt before attaching the binding. your backing can be all one piece (wide quilt backs) or, pieced (keep in mind if hand quilting, extra seams to have to stitch through if you piece your back)
lots of quilt shops offer beautiful wide goods 108" wide fabrics ... 3 yds of this = 108" which will give you plenty for a queen size quilt. all this aside, they are your quilts...use what you like, keep in mind the final product...is it a utilitarian quilt (used and washed lots)?then purchase fabric which will hold up well, is it a wall hanging that probably won't get washed? use what ever you like...
good luck, it's great that you have completed 3 tops, now, time to finish them up :)