Sometimes quilters do not attach, but float the quilt top. I can be laid on top of the backing and batting which can also be floated. But something has to be pinned in place. So that would be the backing.
If you pin your backing to the leaders (both ends) you will have that to support the other 2 layers.
I layer the batting on top of the backing and use just a few pins to hold it in place until I get the quilt top in place.
Actually, I will pin the quilt top to the other 2 layers, until I get the first row of quilting across the top.
After that, both the batting and quilt top hang free.
The reason I do this is to use my batting scraps. I insert one strip of batting across the length and smooth it out.
Then I lay the quilt top on top of the 2 layers - and smooth it into place. After I have quilted the 3 layers for the first row, I renove the pins, but I use a few pins to show me where to stop the quilting ...then lift it up and insert another strip of batting. I save what is cut off the quilt, make sure it is cut straight and then butt the next piece up against the tail end. It is easier than sewing them together on another machine and putting them on vertically. Laying the strips across the width of the frame really works well.
This is what I call floating the quilt top . I suspect some long arm quilters do the same thing, but may not use left-over strips of batting. I make a lot of charity quilts and this is my idea of being conservative
I would like to hear how other quilters do this.