MV DG Sync. Issue

What load are you running at ? if its too low then you could get the symptoms that you are discribing

My guess is that there is nothing wrong with your wiring or the generators.

Every generator (until it is synced) will wander a bit in frequency. That isn't too much of a problem with one generator.

When you put the second generator on, it will sync with the first, but then both of them will wander together. Each will contribute a little to the wandering. That can mean that they wander twice as fast, but perhaps not as far. It also will wander more erratically.

Now, you put the third one on. It's controls are tuned to slowly bring it to sync, but the target is moving faster than it can deal with. Finally, by some great chance, it manages to get frequency and phase together and finally things work out. By then, it's too late.

The solution may be to detune the frequency and phase tolerance a little bit so that it isn't quite so picky about when it declares that it is synced. The other solution is to have a single strong signal which declares what the current frequency and phase should be and have all the units sync to that. Usually, that strong one is the utility.

This is going to be a problem whenever you have generators that are about the same size. If you have one really big one, and a few little ones, the big one will set the pace, and the others will pick up on it.