3 Temporary generators one service

Customer has 3 non fused disconnects from a single utility feed. 200a, 200a, 100a. 240v single phase.
They want to be able to run 3 temporary generators.... one for each.
If I replace the disconnects with fused double throw switches (manual transfer switches) and give them what they need to power up with 3 different generators will they have an issue with the floating neutrals with 3 non-synchronized generators? Do they need to be separately derived systems with switched neutrals bonded in the generators and the generators grounded to earth? It seems like sharing the existing service neutral/ground will be an issue.
 
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