Ok, I've looked it up. The original Defenders were Dr Strange, the Hulk and the Sub-Mariner. Surprisingly the Surfer was not there at the beginning. The original 3 members featured in several adventures together in Marvel Feature #1-3 and Defenders#1 and the Surfer didn't appear until Defenders #2.
I've just thought of an coincidence with the Avengers. Captain America didn't appear until Avengers #4 yet many still think that he's an original Avenger (he has founder status) and the Surfer didn't appear until the fourth issue of the Defenders (counting the Marvel Feature issues) and many today still think of him as an original Defender.
Originally Posted by Rheged
It's because of Titans Three that people think of Silver Surfer as an original Defender, IMO. Valkyrie also joined in #4 of Defenders, and most don't consider her an original member ... though I think of her and Nighthawk as core members, myself. They certainly stuck around longer than Namor or the Hulk or the Silver Surfer.
The fact that the Surfer is considered a fourth founding member is in part due to the Titans Three, but that was the reason why he was one of the three that Strange tried to summon in the first official Defenders mission, against Yon Rogg. Strange actually sent his astral form after the Surfer, only to find him incapacitated after another fruitless attempt of piercing Galactus' barrier around the Earth. Had he arrived only a couple of minutes earlier, the Surfer would have been on that first mission. I guess that, when the Surfer eventtually did join the team, the other members saw this as preordained, Fate correcting its own mistake.