A must? No.
But it probably wouldn't hurt to have three complementary superstars. It depends on what's available. There's a delicate balance when considering the 3rd star or the role players. Neither one is a guarantee and both have pros and cons.
We have two big bodies with a post game, Melo and STAT (more so him abusing bigs with his strength and athleticism than it is an actual post game) both have reliable mid range shots, both can finish around the rim. STAT doesn't control the ball, he's a big, he needs it fed to him...so you shouldn't have situations (in the right system) like LBJ and Wade where one becomes lackadaisical and chills out on the perimeter , while the other walks it up the court and uses picks and drives. So if you give CP3 the reigns to that and let him have fun like a kid on the play ground...I don't even know how to articulate the awesomeness of that. And we all know what Dwight brings...that's a given. Shawne Williams can theoretically serve in the "Mike Miller" role, except without getting paid as much and being less of a douchenozzle (I know his baby daughter is sick, but I still don't like the guy.)
And then on the other hand, we can just add a few pieces here and there and hope we can get enough quality depth to make a realistic run. But it has to be the right players at the right price.
Personally, I don't think cap flexibility should mean as much as it once did. Flexibility is all the rage for teams who need to start rebuilding or just shed bad contracts, teams who are on the upswing and are a few moves away from being actual contenders say "**** it" and bring on all the quality they can get (adding Tyson Chandler right after you re-up Haywood?)
If I have to choose between Beno Udrih (who's game I like), whoever plays SG, Melo, Amar'e, and JJ, or Dalembert, or Turiaf, or guys of this caliber vs. CP3, Melo, Amar'e or Melo, Amar'e, Dwight, and who ever the **** else....I chose option 2 or 2a.