User blog comment:B214/Question and Answers/@comment-50.11.177.49-20130102070717/@comment-33447191-20130102082511

Super Devils are Devils with abnormal demonic powers that far surpass even the original Satans. The only known Super Devils that were introduced in the series are Sirzechs Lucifer and Ajuka Beelzebub. It was mentioned that should Sirzechs and Ajuka show their "true form", they would be ranked in the top 10 in terms of strength of all the religions, a hint to the true powers of the Super Devils. According to Azazel, there is another Super Devil but he has hidden himself. - (From Highschooldxd.wikia). If I had to guess on who the identity of the 3rd Super Devil, I'd probably go with Mephistopheles.

Also just to clarify...

 The Top 10 Strongest in DxD World  In no particular order the following are the among the Top 10 strongest. -Ophis -Shiva -Vishnu -Brahma -Śakra (Indra) -Thor -Typhon (Also Fenrir (before losing his powers)) -Hades -Aten -Lugh

Note: It's said that if Sirzechs and Ajuka were to show their true form, they would be placed among the Top 10. - (Info. from Code-Zero/n0m@n or Shin Code-Zero)