Are two or three finger push-ups a matter of finger/hand or arm strength?
What techniques should I use to learn two or three finger push-ups (if it is finger strength how do I build this up etc.)
Many muscle groups are involved, including hand, arm, chest, shoulder, back strength... it really depends on where your weak link happens to be.
I would suggest first making sure that you can do regular flat-hand pushups with good form. Once you can do that, then try raising up onto all five fingers. Once you have those down, you can start taking fingers away.
If you find that it is too difficult to do many on your fingertips at first, try going to your knees and doing pushups from your knees with your fingers. The point of these isn't to tax your chest/arm/shoulder assembly, but rather to train your fingers to hold up the light load before you do full pushups on them.
(I assume you can already do a fair amount of "normal" pushups with good form)
The main limiting factor are probably your fingers - while you have muscles in most of your body otherwise, fingers are mostly tendons and fingertip pushups put a lot of stress on them. Unfortunately, tendons develop much slower than muscles and so whatever issue you might have, sooner or later it will boil down to tendon strength.
To train your fingers, at first you'll just want to hold the "up" position on your fingertips. Try to spread them around as much as you can, see this picture. Be very, very, very careful as it's easy to mess up your tendons and that's a long recovery period later. Just try to do it each day (or even a couple of times per day) and have patience; it could take more than a month to get the required tendon strength. Stop if you feel any kind of pain. Once holding it is effortless enough, transitioning to an actual pushup is easy.