3 separate employers -
Corporation 1 - 300+ employees, 100% of stock owned by John
Corporation 2 - 5 employees, 100% of stock owned by John
John's Sole Proprietorship - 4 employees

In the past the two corporations have shared benefit plans. For example, on the self-insured dental plan, Corp 2 is listed as a Participating Employer. Same with the 401(k) plan. I am not sure how the insured medical plan was set up, since it has been in place for like 25 years, so perhaps it was not done properly at the time, but I believe it was, and that the insurer was aware that there were 2 separate corps involved in the plan.

Now we are asking about adding the 3rd company to the plans and being told we cannot do that, they would have to have their own small plan. Is this correct? Is there any way to have these 3 plans all together for all benefit purposes? Do we have any options?

on the welfare plan side, the issue seems to be related to if the companies are related or is the arrangement a MEWA that would require registration with the state.

Well the companies are related, in the way I wrote, in that they are all owned by the same person, and they share management personnel.

I am not familiar with MEWAs, other than having heard the term. Is it something that is easy to set up and administer? Can anyone do it? Would that be a good option in this instance?

I can see no legal reason why the 3rd company can't be added. If they did it for 2, why not 3?

It doesn't make sense.

Did they give you a reason why?