Yes, these are tough rivets for sure. I squeezed the bottom two but used an offset for the top one. Not easy. Congrats to you for going the extra mile and not just assuming "it'll be OK and nobody will see it". Those are the types of places I always look at when checking out somebody's project.
Regarding the offset. I too use something to hold the rivet set from rotating in the gun. As czech6 said, duct tape works well is cheap and fast. I have found the key to using the offset is to keep significant pressure (hand pressure not air pressure) on the gun and make sure it's at a right angle. Seems like common sense, but when I finally figured out that to use an abundance of pressure on the gun, I no longer have any issues or fear the offset. In fact, I use it frequently.