When you say you have 3 straps, is this 3 each side or 3 in total?
Mine has 6 (as per your picture), the inner 2 on the back are Velcro, the others aren't. I got the non-Velcro ones on mine to hook onto trim at the side and rear, but didn't find anywhere to hook the Velcro ones on to. However, the 4 that are hooked on look as though they will keep it rigid and on the car.
The job is easier if you raise the roof a little.
I am super-impressed with mine. Drove to the station this morning with roof off. Temperature was 5 degrees but with heater turned up and toasty bum option on high it was very cosy.