They should let one car teams participate and allow technical partnerships between such teams.
If Ferrari want a 3 car team, they can partner with a 1 car team and provide them with a car. They can use the small team to share the burden of testing new parts without affecting the preparations of the main team. In return, the small team gets a car that is actually on par with the established teams, giving a driver new to the sport a chance to shine and the team to attract sponsors, because they aren’t a back marker.
It’d be just like the 70s when there were a slew of small teams who were running Tyrrells, Brabhams, Shadows etc.