I have a Mach3 unit with 1 capacitor 35uf 370 VAC and a fan capacitor, unit trips breaker after cooling RV when system starts up again, shouldn't there be a run, start,fan capacitor? only have 2 located in unit.

If it trips the circuit breaker as soon as you turn the FAN on, suspect either a bad fan motor or a failed fan motor start capacitor. If you are able to turn the fan on but the circuit breaker trips as soon as the COMPRESSOR starts, it is possible that the compressor is seized.
If, on the other hand, the unit is tripping a GFCI (Ground Fault Circuit Interrupter) breaker or GFCI utility outlet, it is possible that neutral and hot are reversed at the outlet into which the trailer is plugged (happened to me this year at an RV park).

the three post capacitor has c for line voltage in, the f or fan is for the outdoor motor,and com or comp goes to start winding compressor.youll have to follow wires from indoor motor to find capacitor. to find wires to outdoor motor to capacitor you have line voltage and capacitor wires .look for schematic it would help or send colors of wires from both motors and compressor common,start .run.

bad compressor relay on control board. The relay is not opening the circuit, allowing the compressor to stay running when unit is turned off. This overheats the compressor and opens a thermal disk shutting the compressor off. When thermal disk cools down, the compressor restarts under load and trips breaker.

Sounds like possible fan capacitor failure. If your not familiar with capacitors, please find someone who is, as they can give a fatal electrical jolt if not handled properly.

Does it trip when fan only , or when compressor for cool cuts in? Sounds like capacitor perhaps . Get back to me with model number, and whether it's a ducted system, or blows out at ceiling assembly.

This is a fairly common problem with older AC's, especially ones on a RV. As they age they start to break down (electrically/mechanically) a little, and therefore when the load is the greatest, i.e. the heat of the day, they will pull a little more amps (than what they and the breaker is rated at) and trip the breaker. This is a pretty good indication that either the fan motor or the compressor could be on it's last legs.
Not exactly what you wanted to hear I bet?
There is some other possibility's, however.
1. if the system has been opened (a easy way to tell this is if - 'service ports' - have been added in order to charge the unit) - it's possible that whoever 'charged it' - put just a little too much refrigerant (an overcharge unit will pull more amps) in it. The best way to tell this is to put gauges on it and check the running pressures - during the heat of the day. It's possible that someone knowledgeable with AC's - could 'fine tune' your charge and eliminate this problem. Note: obviously if you do not have 'service ports' then an overcharged system is not your problem.
2. The breaker might be getting 'weak' and tripping early. I don't think of this as a good possibility, (since breakers are usually good or bad) - but, you might want to think about replacing the breaker. Also, it's possible that the breaker is not rated high enough to handle the AC. I would check with an electrician and see if he/she thought your breaker was sufficiently sized right.
3. you might have a fan or compressor - 'capacitor' (silver or gray oblong electrical devices) going bad or already bad. This is a remote possibility but if upon examination you see one or both of the capacitor's with a 'bulge,' then I would swap it. Note: Usually capacitors that go bad - have a bulge - but they can be bad and not have a bulge.
hope this helps.

Usually just listening closely will give you a rough idea where to look for problem. When you have the fan on and switch to cool, (with temperature turned down low obviously) do you hear any type of "moaning" sounds from AC unit, or see lights in RV dim slightly indicating that compressor is trying to engage? Is the breaker tripping when compressor tries to engage? Do you hear a "click" from AC unit when switched to cool? If none of that happens, then I'd start thinking thermostat, or compressor relay. Normally if it's a faulty capacitor, the compressor will try to engage but will make a "moaning sound" and usually trip the breaker due to excess current draw. Someone experience in electrical should be able to test relays etc, but whoever does work on it, should be aware that those capacitors, as well as hydro in general are very dangerous to work with. The ("zap",- Shock) that you can experience from a capacitor or hydro, can be fatal.

Check outside unit to see if compressor is running. If compressor runs and fan does not. Check to see if fan is hot. If it is you have stuck fan motor,bad capacitor or bearing are out. Capacitor is located in wiring compartment,it is a silver canister with 3 terminals at top. Check to see if swollen or oily. If it is bad look on side of capacitor and it will tell you size. It will say 35uf 5uf 370 or 440 something like that. But that is the size. Rus

Check the fan relay or contacts first. If the compressor comes on then your fan may be locked and will need replacing.