WRONG.
All heroes inside there will be included under the 3 heroes max that you have in a melee.
For example, if you had Beastmaster, Archmage and Firelord while they are still there, then you can't get/build anymore heroes.
Another example. Supposing I made 4 new heroes called B, A, D, and C, and I put all of them there, I would be able to only have 3 of these heroes at a time.
BUT if your hero is not under that Constant, you are able to build other heroes.