I agree 6 kw is too small for 3 AC units running, particularly since 50A units tend to have 2 AC units in them. I would estimate somewhere around 10 kW would do it. You can probably rent a tow-able diesel unit that would serve the purpose well. They have some that are incredibly quiet for their size.
I believe you would also save fuel; 1 generator running under varying load is likely far more fuel efficient than 3 or more generators running.
I disagree that you would need a 30 kW generator as there is no way your going to have all 3 RVs pulling the maximum their service will allow all at the same time. Most RV parks couldn't even support this type of scenario.
However, likely, when you start looking at rental diesel generators they will be in the 20 kW to 30 kW size range.