Are the PAs in an affiliated service group with the partnership? (Probably yes - this is the classic ASG scenario, and it would explain why the three doctor partners are able to participate in the partnership's plan even though they are not employees of the partnership. However an ASG determination is highly fact-specific, and the standard advice to consult an attorney applies.)
Assuming this is an ASG, then the one doctor could adopt his own plan, but it would have to be aggregated with the partnership's plan for testing. Keep in mind that includes not just benefits (ADP/ACP, etc) but also availability of benefits, rights and features, so he can't for example have an investment in his plan that isn't available to the employees of the partnership.