Stamford Alt. Parallel Running

Hy,

I have a good question for specialists. I have a plant consisting of three gensets. For two of them the engines are KTA 19( Cummins) but for the last one the engine is bigger KTA 38 ( the reason is to connect by a clutch a fire fighting pump) The alternator side are all three the same Stamford HC.. about 500 KVA. Well gents with the No.1 and No.2 connected to the bars, when I try to start the No.3 ( the one with the fire fight. pump) after couple seconds of running in idle/run speed (750 rpm/1500 rpm)the two gensets connected to bars start overspeeding and tripping.
The main problem is that I don't have any king of drawing. All king of information in this matter is highly appreciated.
I have an idea to try to disconnect from the parallel operation condition and to leave alone No.3 not to be connected to the bars any more or to parallel operation.
For that I need also ideas how to do it.
All three are connected for parallel operation with the quadroop CT system and the synchronization and load sharing is made by a PLC (Modicon STX)
With the hope that you share from your experience I wait for your opinions.