First you have to test for leaks. With the information you presented you probably ought to find a good local A/C shop that can sniff any leaks for you. Do not throw any more money at the a/c system until you find the leaks AND detemine why the system went through 3 compressors. It takes a special tool to do this. The sniffer looks for hydrocarbons and sounds like a geiger counter. Find all the leaks first, repair the leaks with new parts, new o-rings on the hoses etc. Read about the flush agents.

Once you determine the condition of the system, lets talk about compressors.

__________________
84 300DT Puke Yellow. Totalled after 438,000
84 300DT Orient Red. 169,000 (actual mileage may vary)
2002 Explorer EB (wife's)