I'm not a medical professional of any kind. Nothing I pot should be construed as medical advice. What I post is just based on my own experiences at best or idle speculation at worst. Always ask your doctor and follow his/her advice.
I believe if you had good lasting erections before your RP you should get good results with injections. Meanwhile, get a VED (vacuum erection device AKA pump) and start using it to keep blood flow going to your dick. Try it for sex with medical grade constriction rings if you want but at least use it for therapy.
I'm
STILL not clear on your post. You say you tried 3 shots at 1.5, 2.0 and 3.0. I'm unclear on what those numbers represent. Are those units as displayed on the syringe?
Three shots of what? What does the script say the mix is? The mixes can vary in potency dramatically.
What you do with your boner is up to you. The injection should give you an erection without any stimulation but stimulation should help too.
If you were able to get and maintain an erection before your RP, the injections should work for you. You and your doctor just need to determine what mix and dosage works for you.
Try this video:
https://www.youtube.com/watch?time_continue=10&v=DHc6NI5rTDw
RRP 2011 Mayo Jacksonville, Dr. Wehle. Non nerve sparing. C in margins. Radiation 2023, V.E.D, Viagra and PGE-1 (80mcg/ml) injections @ 7 - 16 units. Originally Edex20 or 40, then compounded PGE1 due to cost. Inject. 14 yrs. It works. FL Treasure coast.