using tubes as wheel spacers?

When wheel studs are installed in a normal application and torqued, the load on the stud is compression due to the torque of the lug nut and pure shear from the weight of the car. This shear is reduced if the wheel center rests on the axle register.

With your tube spacers, the studs will have a bending moment applied as well. This is because the tubes can rock. Not only that but the bending moment will oscillate with the rpm of the wheel leading to fatigue.

Five separate spacers are different than a single larger spacer with five holes due to this ability to rock.

Three inches is a lot, but if you are determined I suggest finding or fabricating a single spacer with five holes.