The compressor on my 3 ton unit cycles on for a minute or two and then off for a minute or two, for long periods at a time. The blower is not cycling on/off because of the thermostat setting ( themostat at 78, room temp at 84). The air is cool, so it's not very noticible inside the house that the compressor is constantly cycling, but this must be bad for the compressor. Any advice on what is wrong?

Sounds like it could be low on refrigerant. Shut unit down and call a qulified tech to service unit. Running it like this is hard on unit and will cause more expensive repair. Good Luck

It may be a safety switch cycling it, a low pressure switch will do this as well as a high pressure switch, this is when if you have a volt meter you would check these switches.
The low press. sw. cycles the unit if the system gets low on refrigerant and the high pressure sw. cycles it if the head (discharge ) pressure gets to high, such as a bad fan motor or very dirty condenser.