From the outside looking in i think the tri state area can more than accommodate 3 MLS franchises.
Something about the Cosmos still feel right and a sense of nostalgia.
NY RedBull is the one that may have to shift their fan base to a more New Jersey centric base, their stadium is already there and it may mean possibly a name change? I see NYC FC can be the city centric franchise and the Cosmos if it can work be more suburban in its model, or vice versa.
The cosmopolitan nature of NY/NJ/CT can sustain everything. A nice mix with the outlying Philadelphia and New England franchises as well.