I think/know my husband must be one of the very, very lucky ones in this area, (incontenence is another story, still working on that). His surgery was in July, an open radical prostatectomy that only left one nerve bundle. He was started on viagra as soon as the catheter came out, 25mg at night. We were told that this was prescribed to help blood flow return to help with erectile function. Three weeks after the surgery, we were able to be intimate! I'm not kidding, three weeks! Now mind you, it wasn't 100%, but enough for penetration which we were both very pleasantly surprised to say the least, it practically brought tears to our eyes. Things have only got better from there. Still not 100% as what it was but probably 75% anyway which I think is a miracle, especially from what I've read about how this can be a real problem area. I know we're very fortunate and I can't tell you how thankful we are.
I think it is important for couples to know that all is not lost, even with only one nerve bundle. Also, Swim is right, even if things don't come back by themselves, there are many other options available that can make it happen and that we were fully prepared to try. I do believe though that three weeks is quite early, I think I'm just married to a horndog but then again, it takes one to know one J/k! On the other hand, his age and great physical condition probably has a lot to do with it to. I don't know, I just know that we really, really lucked out in this area.
Don't give up and just give it time. We wish you and all the rest the very best,