For:

two 3-tier cakes (for one wedding)

6, 8, 10" round cakes (approx. 4" high each).

Or how much fondant for one 3-tier cake (6,8,10) if it's easier and I'll double it.

I really don't trust my guestimates when it comes to fondant, even looking at the Wilton Chart. I always end up tinting way too much.
There will be a lot of decoration, but mostly gumpaste, if that helps.

Thank you.

Check this out:

http://earlenescakes.com/Fondantchart.html

Thank you.

The site karateka posted looks great and I like how the author indicates why you need to prepare more fondant than actually needed!

I saw that the cake sizes you specified would require you to prepare 7 1/2 pounds yet only need 3 1/2 pounds to cover. I think if you are using the same color for all three layers of one cake, you could reduce the amount you prepare as you would be able to use the scraps from one cake to help cover the next. However, I'm not suggesting you substitute my judgement for your own

Just so I undertand.... they are saying I will only need 3 1/2 lbs total, yet telling me to tint 7 lbs? Is that right?

I suggest you look at the site...it's a good resource. But like I said, because you are working with several cakes, you likely would not need as much extra.

She really puts it well. I would prepare...ummm...maybe 5 lbs instead of 7lbs for the one, 3 tired cake. Cover the largest, middle, then smallest. While extra can be annoying, not enough is maddening