I am making my first 3 tier, 8in, 6in, 4in. I have the cardboard dowels and cakeboards, will this be sufficient for this cake??
Your input would be greatly appreciated....

I suggest you watch this superb video by Sweetwise, Inc.

http://www.youtube.com/watch?v=KdzLN5hjulo&list=PL178BA8A7498017F9&index=5&feature=plpp_video

Once you have watched the video, you should have a very good idea of whether or not your preparations have been sufficient.

Assuming you're doing a straight stack?

I use a ruler to check my distance between tiers since being a little off can mean a leaning cake. I measure off my dowels by inserting one, marking it off and then removing so I can cut the rest to match. I dinged a cake once cutting a dowel in place and had to patch it so lesson learned!

Apti's video link is a great one =)

You'll do fine! The first one is the scary one

Yes, cardboards and dowels (I use plain old drinking straws) are plenty sufficient to support this cake (I use only this for cakes up to 6-9-12-15")