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.

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.

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.