This may seem like a really stupid question to some but, are the mags interchangeable between Ruger Mark I,II and III models? I've just picked up a Ruger Mark I as my first handgun and wasn't sure if I needed to pick up model specific mags.
No, not completely.
The Ruger Mark III's have the addition of a magazine disconnect and the magazine release notch differs (Heel vs button). I also think that some very early Mark I's had the little magazine slider button (Not on the frame, on the actual magazine) on the opposite side. You should be okay between I and II because I think you can swap over the magazine slider button if necessary.
Someone with more experience than me can confirm this for you.