Thanks to Dread:
Guy at the top right looks like a strong guy, doesn't this team have enough muscle?
No problemo.
Yeah, between She-Hulk and Colossus the team does have enough muscle. Then again, I've seen Avengers rosters with Hercules, Thor, and Wonder Man on them.

Heck, the Mighty Avengers have Ares, Sentry, and Wonder Man (Ms. Marvel is a decent tanker herself). He could just a bulky guy.
I am thinking at least one may be Valkyrie. I mean, why not? She, Nighthawk, and Hellcat have been Defenders for the longest time, sometimes longer than the founders were.
They seem to be getting top talent on the covers, as this looks like Yu.
As for official hints:
Joe Casey said:
: We're definitely adding three new members in this issue. In terms of hints, let's see... I can tell you that one of these characters has a long and sordid history in the Marvel U., plus he's worked with Nighthawk before. Another one was actually a Defender at an earlier point in his career (even if it was only for a day). But, the main thing for me is that each of these characters adds a new flavor to the series. For now, let the guessing games begin...!
He claims 2 out of 3 are male by "he". But, who knows. It could have been a slip.
The "Defender for a Day" hints to the story in DEFENDERS #62-64, when Defenders associate Dollar Bill exposed the team and their lax standards on TV and a slew of heroes tried to join, although most quit and never associated with the team again after one hectic mission.
The "Defenders for a Day" who are still alive are:
Captain Ultra
Nova (Richard Rider)
Captain Mar-Vell (sorta)
Falcon
Havok
Hercules
Iron Fist
Tagak the Leopard Lord
Paladin
Polaris
Prowler
Stingray
Now, Nova, Havok, & Polaris are off world right now. Mar-Vell's status is questionable given what is happening in his own mini now. Iron Fist is already on a team right now, and Hercules has his own title (for the moment). That narrows the list a bit.
(Oh, and the dead "Defenders for a Day" are White Tiger (Hector Ayla), Jack of Hearts, Torpedo, and Black Goliath.)