Marineair chillers replacement or repair

5 tons = 60,000 BTU

5 tons each x 3 = 180000 btu's total.

Sounds like you have 2 units with refrigeration leaks.
If you see oil, then you have a leak.
Did the tech find the locations of the leaks?
You can use soapy water and spray the lines, joints and the tube bundle shell.
You may also have a leak in the tube bundle.
Even if you have to replace the tube bundle, it shouldn't have to be overly expensive!
Repair the leak, replace the filter/drier, and charge the system..
Don't forget to descale the raw water side once in a while, to prevent frozen tubes.

Are you saying that all of the chillers are 2010??? I would find a new A/C guy immediately. Sounds like he just wants to sell units. Chillers that are properly installed should last 10-12 years usually. 6 years is very premature. I would absolutely NOT remove the working chiller. The question is why are these chillers failing so early. I would check freon pressure in the one that is running and see if it is overcharged. The chillers are fixable, but need to determine where the leaks are. It might be possible to make one good one out of the 2 that are not functioning or repair both of the non functioning ones. Retail price on a MCGx60 is $11,770.00 per Dometics 2016 price book. If you want smart start, which I'd highly recommend retail price is $12,235.00.

I replaced both my chillers season before last. I don't think they were quite as many BTU's as yours but I can't recall the actual size. They are Dometic Compact units. One was still working but the reversing valve on the other was stuck. Mine are difficult to get to and have to be pulled to service them. Since they were original to the boat (1994 model) I decided to replace both. The units were about $4,500 each for the parts.
I'm in a similar climate to you but keep in mind these units also provide your heat unless you use a separate heating system. I used Cliff's Marine Service out of Seattle as they are a local Dometic dealer and Cliff is pretty knowledgeable. Installation was pretty straight forward as it is just reconnecting some pipes and electrical.
If you only got 6 years of service on yours I would say something else is wrong in the system to cause failure that early. You have to be careful that your piping loop doesn't freeze in the winter unless you have the proper solution. I drained mine as it only had water. Filled it back with the recommended glycol solution and bled all the air handlers. It was a bit of a pain to get it all bled but worked much better than ever before once it was all done. It was also recommended to regularly cycle my units and switch them from a/c to heat a few times so things don't seize up.

Generally you can bleed all of the air out from the highest unit in the system, like if you have a flybridge mounted air handler.

I have a similar system set up (although smaller total BTU) and had an experience that I wanted to highlight. Got repeated Low Pressure alarms. Would refill Freon and would alarm again within a few days. Discovered one of my chillers had multiple pin hole leaks that made repair impossible. The culprit turned out to be that the unit was NOT bonded to the whole boat bonding system. The adjacent unit, which was adequately bonded, has continued to operate without an issue. Had to replace the one unit ($5-6k +labor). These units definitely should last longer than 6 years. Most industry people will say 10-12 years. Similar to a home heat pump. Although...5 years in FL is like 10+ years everywhere else. Hope this helps!

Inspected??? By whom? The tech should be able to inspect them on board as long as access is decent. Gauges should tell you how they're running.......Again, you need a different tech.

Agree with Capt J and Pascal. Are you having to add fluid to the piping loop? Pressure loss can be coming from lots of places in the system. Honestly, a fluid leak at the chillers should be easy to spot but is one of the less likely places for a leak. As suggested, any of the air handlers, bleeders or fittings could be an issue. You also could have a circulating pump that is getting worn and just not building pressure. Many of these systems have the ability to add water to the loop direct from your fresh water system and there would be an isolation valve and a check valve or back-flow preventer so the glycol mixture can't get back into your fresh water. You might want to check there to ensure you are not leaking pressure at either of those valves. Last but not least, have you verified the gauge you are relying on is accurate and reading properly?

Does it hold pressure once it goes down? You might have to add for a while in 30 second bursts as the water/ coolant works it's way through the system, especially as the pump is running. Quite honestly, you need a more knowledgable tech.