Wendonai is described and statted in
Lost Empires of Faerun, page 54. He's a balor with six levels in the Fiend of Corruption prestige class (Fiend Folio).
Errtu was described and statted in the 2e
Villain's Lorebook, which you can download
here for free (though it's barely worth the effort). He was presented as a standard balor tanar'ri, unexceptional in every way. His intelligence was slightly lower than some 2e balors, but he was still a genius. Except vaguely noting that he has a grudge against Drizzt and his friends, the book didn't even bother to give him a distinct personality. It mostly just recycled the generic balor information from the
Monstrous Manual. Even the illustration was recycled from there (and doesn't fit his description in the novels). The book did not say whether his hit points were below or above average for one of his race, but given everything else "average" seems a safe bet.
Ndulu is mentioned in
Champions of Ruin, pages 130 and 132. He's a 32 HD advanced balor, formally the master of the Citadel of Conjurers. He forged the Moaning Crown of Ndulu from the souls once trapped in the Crown of Narfell. He was banished back to the Abyss by the paladin-prince Nord.
Ter-Soth appeared in
Throne of Bloodstone. He was a normal balor with above-average hit points. He led a brigade of 100 glabrezu in the service of Orcus.
Balor himself appeared earlier in
Throne of Bloodstone as a servant of Demogorgon. He had average hit points for a member of his species. While unexceptional statistically, he gets the best line in the adventure: “Do you tire of mortal life so much that you seek to end it at the hands of Prince Demogorgon?”
There's more information on these balors in their
Wikipedia entry, which says that Ndulu is mentioned in a few different places.
Update:
Errtu has a Wikipedia entry as well. Apparently he rules his own layer of the Abyss! Must be a minor one.