They ARE Severums of some sort, however, it's pretty difficult to distinguish between the Severums that are available in the hobby today. I can tell you that they most likely are not Heros severus as these are rarely found in the aquarium trade.
They are probably under the common umbrella of 'Heros efasciatus'. (Heros Notatus, Heros Sp. Rotkeil, Heros Appendiculatus...to name a few). Try doing some research on the internet about the differences in color and shape of these fish.
I can tell you though, that the yellow labs and Sevs should not be together. Also, that the Sevs grow quite large. So 3 of them (along with the yellow labs) should not be tankmates.
Do some research man, I love my Sev so I highly recommend keeping at least one (if not all) if your tank is large enough!
I love this stuff!