I don't have experience with the R3, but I did play with some films in low light at fast speeds last year. So, I can say, that from my [limited] experience ...
Neopan 400 pushes, but not well. I've gotten it to EI 3200-ish, but the shadows are completely gone much past 1000 or so if you're shooting dark scenes.
Delta 3200 is great [IMO, best] at 1600. I think that 3200 is a little high for this film if you want to retain shadow detail. I've had acceptable to good results with D-76 and great results in Microphen. It's grainy, which can be an issue in 35mm, but since you're shooting 120, you'll be fine.
Tri-X 400 is, IMO, great for pushing. I've had it as far as EI 6400 (in TMax RS) with surprising results. It works well in almost all situations at 1600. I would use something other than Diafine for it, though. D-76 (or ID-11) works very well for Tri-X at 1600, and is easy and inexpensive to obtain.
I'd reccomend the Delta 3200 @ EI 1600 or Tri-X at EI 1600. I wouldn't bother with the Neopan 400.