I've done mixed fabrics memory quilts before. If I do sashing or borders, I prefer to use either the same fabrics or cotton. I don't like too many different fabric types in my quilt (totally my personal choice). Have you considered taking off the 3" fleece border and adding one piece of fleece to get it to the desired size? Or maybe add another piece of fleece (same or different color to get it to the right size? I think seams in borders tend to disappear when they are quilted. Just a suggestion....HTH!