I also use an R3, used it at NJMP last summer. I can put it on myself, it doesn't require someone out side the car to assist w/straps. And it doesn't require any modifications to your car's existing belt system.
I attach it to the helmet, then put helmet on, then buckle R3. I bought it 4 years ago so there may be better systems out now, I haven't looked.
It is self contained (doesn't rely on shoulder straps). So you can use w/3 pt system. And you can hop from car to car w/it on (use as a passenger). There is almost no neck rotation tho, so you must have mirrors adjusted perfectly. It takes abt 5' to get it on, and another few minutes if you are using arm restraints, then the 5-6 pt harness, so buckling up is a process. So I usually start abt 10' before griding. Once out on the track, it is invisible, and adds a sense of (hopefully real) protection.