why do u think duncan waited till season 3 to kiss gwen?

Becasue he was waiting to break up with Courtney but he couldn't wait any longer. So he kissed her in London.

Season 1:Duncan liked Courtney
Season 2:Duncan was friends with Gwen,but still liked Courtney
Season 3:Duncan somehow lost intrest in Courtney and started to grow feelings for Gwen so then he kissed her.

Well, the only people that can answer that are the producers, but if I had to say, I think it was because he was afraid of ruining their freindship, and wanted to make sure he liked her too. Example of Duncan liking Gwen before he kissed her is the scene in "Walk like an Eygiption" when they walk out of the bus:

Gwen: Are there assigned seats? I.E. can I have one not behind Heather's pony-hair ponytail?

Heather: My extentions are HUMAN hair!

Duncan: You learn somethin new every day.

*Gwen laughs*

*Gwen and Duncan look at each other in a dreamy way*

See what I mean?

he probly wainted because he was still with courtney

cuz he wanted 2 make sure his feelings were clear 4 gwen.

Well in season 1 he was mostly after Courtney! We all know that.

But then in season 2 Duncans friendship started with Gwen but Duncan still had feelings for Courtney.

When season 3 came, Courtney was just a bitch the whole friggin' time and Duncan saw that Gwen was more mellow and not as Uptight as Courtney so he relized that Gwen was the one for him and he kissed her.

in tdi he was dating f****** courtney
in tda gwen was with trent
in tdwt (my favorate) they started dating :)

In season 1, Duncan was with Courtney, but did have common interests established with Gwen.
In season 2, Duncan befriends Gwen and they actually have a really good friendship. (I personally feel like Duncan started to like Gwen here.....but he still dated Courtney because he still liked Courtney a lot)
In season 3, Duncan is looking at Gwen as more than just a friend. The first episode alone hints at it. And then boom, he comes back......after his long break away from Courtney, and then after being bombarded by Courtney when he gets back about him "abandoning her," he realizes that Gwen is the perfect girl for him.

BOOM........he kisses Gwen. ;D