Hi Lucy,
In years past, the old Nikon CP series were untouched for macro capabilities, but in recent years things have changed a great deal in the design of digicams with a move toward faster and larger lenses with much more zoom than in years past.
Truthfully, for doing what you have in mind, none of the new digicams are really ideal unless you use accessory lenses. With the new longer zoom lenses, macro shots of tiny insects have become a much more specialized field.
Nearly any of the three cameras you mentioned plus the Canon Pro 1 all are capable of taking excellent macro shots with the right macro adapter lens. But more important than the small differences which reviewers found in such things as distortion levels and edge sharpness is the ability to actually get the shot in a poorly lit environment. Both subject movement and camera movement play into the scenario strongly. Of the cameras you mentioned, the Minolta A2 has the "cure" for one of these issues which is camera movement. With the stabilized lens, you will be able to get ambient light shots with a slower shutter speed meaning that half of the problem issue (camera movement/subject movement) is pretty well covered.
Either way, you will most likely want to invest in a ring flash for your camera. The ring flash will help greatly with shutter speed but even more importantly with being able to shoot at small apertures. You really need to shoot at F8 with any of these cameras to get the depth of field necessary for tight in shots of tiny insects.
Were I embarking on the journey you have chosen, my personal pick of the three for detailed close-up macro work would be the Minolta A2 because of the image stabilization.
One of the finest macro photographers frequenting the forums today is a nice chap from New Zealand who goes by the handle of NZMacro. Danny has a unique ability to wrest the absolute maximum from any camera he uses for close-up detailed work, and his present tool of choice is the little Panasonic FZ10 with which he gets fantastic macros. You may want to go over to Phil Askey's forum and put out a query and get some advice from Danny. Not so much from the standpoint of which camera to buy, but more from the "how to improve" one's chances for successfully shooting the little critters.
Best regards,
Lin
