Which is better to make a bow from  ash,  maple or hickory? And would they all need backing?

hickory, hands down.  Although, I'm currently making one out of maple, and it's going okay.  gonna start tillering tomorrow.  Haven't tried ash yet, but I've heard it's right up there with maple and hickory.

Actually, if you've got access to some osage, that's the best.

Any of the three will make a good bow. For me, hickory is more forgiving but other folks make great bows from maple(hard) or ash. If you take your time, use the proper design and not force the tiller you will do fine with any of them. Heck, try them all and see which works better for you.
 With a good back ring, you shouldn't need a backing for any of them. I only back any self bow if the back ring is iffy.

I've made bows from both hickory and ash, hickory is more forgiving but ash can make a great bow