(1) Billy is Cobra Commander's estranged son from the comics. He once attempted to commit patricide at the behest of the Baroness and Major Bludd - both of whom later abandoned him. He has undergone extensive ninja training under the apprenticeship of Storm Shadow (who has almost assumed a role of surrogate father to Billy). He has lost an eye and a leg to an explosion in a motor vehicle accident which left him briefly comatose. He was buried alive in a landlocked freighter on Cobra Island at a whim of his true father, but escaped death together with Zartan and Firefly. We get to see his mother (Cobra Commander's ex-wife) in the Marvel comics, with whom Billy is still on good terms. He's still around in Devil's Due after coming out of seclusion in Scotland. He goes by William "Kessler" these days.
(2a) A young clone of Serpentor from Devil's Due comics...aged nine or ten. Recall that Serpentor is cloned from the greatest military tacticians/leaders of global history. The name is a reference to Alexander the Great, the young Macedonian empire-builder who conquered the known ancient world (for that time).
(2b) A separate character named Alexander is the illegitimate son of Destro. He is in his late twenties or early thirties and despises the Baroness, whom he believes prevented his own mother from being able to have a meaningful relationship with Destro. He is featured in Devil's Due comics, and actually usurped his father's position in the first story cycle. He often wears a silver ceremonial battle mask and desires to gain more power in Destro's new Iron Grenadier forces. He is not the equal of his father, as he lacks the military genius, the sense of honor, and the charisma of the elder Destro. He is both ruthless and aggressive. It remains to be seen what role he will ultimately assume in the newer GI Joe continuity.
(3) Yet another clone of Serpentor from Devil's Due...aged about sixteen to eighteen (my memory lapses on the specific ages of these clones). Hannibal is the oldest of the surviving clones (the others are roughly equivalent in age to Alexander). The name is a reference to the Carthaginian general who assaulted Rome during the Second Punic War.