How to suspend rebar cages for piers
Most inspectors will allow 3" dobbies, but really you should use 3" chairs to keep the rebar 3" clear off the bottom and sides. (The reason your engineer specified 3" is because of moisture protection and chairs allows less moisture than dobbies.) 

I'd wire the steel in place at the top using a wood frame cage set 3" in on the sides and wire the cage to keep it from falling side-to-side. Don't wrap the wire around the steel more than once. Then after the concrete is installed, you can pull it out so no moisture penetration points. 

By the way, I'd use 3/4" minus aggregate because of the closeness of the ties on top. 

Also, you can use the wood frame cage to guide you when you set the 10" square steel plate. Make a template and it will fit perfectly in place when the "mud" is still wet.