Hi djones and welcome to the forum.
This isn't really the best thread for this discussion but oh well!
I'm not a medical professional of any kind. Anything I post is at best based on my own experiences but at worst could be based on delusions or hallucinations. Remember, any internet forum advice could very likely be worth exactly what you paid for it, or worse. Always ask your doctor and follow his/her advice.
My first impression is that as you had two "successful" injections before your failure I'm going to guess that you missed the sweet spot with your third. If the juice doesn't make it to the correct spot it won't work.
It could be that your current ideal number of units is somewhere between 10 and 15 units. When you're at 10 units adding another 5 is a pretty big bump. If you were pretty good at 10 I'd have thought maybe 12 to 13 would have been the next step.
I believe most doctors discourage taking both Viagra and an injection together due to the risk of the priapism, such as you feared you were having on the second shot. Therefore I certainly wouldn't recommend the practice. However, I combine Viagra with a mono-mix injection fairly frequently. Bear in mind that my ED is a result of a RP and is fairly extreme. Viagra gives me NO erection but it does enhance other methods like injections or a VED.
So to not answer your last question more directly (non-medical professional speculation) I'd have to say the Viagra contributed to the reaction but the increased tri-mix volume likely contributed as much or more. You'll never know if you're throwing more than one variable into the equation at once. Don't do that.
It does suck when doctors don't educate patients adequately. I can relate.
R.R.P 2011 Mayo Jacksonville, Dr. Michael Wehle. Nerve sparing - badly damaged. C in margin. V.E.D, Viagra and PGE-1 (80mcg/ml) injections @ ~ 14 units. Originally Edex20, then compounded PGE-1 - cost. Inject. ~ 9 yrs. It works. Treasure coast of FL.