apartment therapy changing the world, one room at a time

Here's the thing: if you're trying to make a set of 3, I don't think it will work unless the 3rd chair is 100% identical. Having a chair that looks ALMOST the same but isn't may not be the strongest design decision.
So if you can't match the 3rd chair to this one 100%, I'd think about working the contrast. Maybe a similar shape, in a complimentary color? So you embrace the non matching- ness rather than try to cover it up. As Diana Vreeland said, "If you cant fix it, put a flower in it". Or something to that effect. Good luck!

check out my post on 2 orange chairs that would work in the sense that they are orange and retro, but are different enough that they wouldn't look like you tried to match but failed:

http://honeyliving.blogspot.com/2009/08/orange-you-in-love-with-these-chairs.html

PATRIK from Ikea is similar, but is more suited for the office than the living room. I also found it to be not entirely comfortable, and possibly too narrow in the seat for some:

http://www.ikea.com/us/en/catalog/products/70074950

It's clearly vintage - Keep looking on eBay and the fleamarkets and be prepared to reupholster them all to match.

Search on ebay, craigslist for - four prong base swivel chairs, retro, vintage, etc.
You could get another two chairs that are different but of the same decade, look these were posted here-
http://chicago.craigslist.org/chc/fuo/1364510082.html

http://losangeles.craigslist.org/wst/fuo/1365369293.html

The base is definitely Fritz Hansen or KNOLL. I know I've seen this chair before but I can't remember exactly where.

Agree with mskk and bepsf. Even if you find an exact match on a vintage chair, the fabric on the 3rd chair will have worn differently, and the color might be off.

The PATRIK chair from Ikea is awful. I can't even sit in it for more than a few minutes.

that is an amazing chair!

Eurway.com makes some similar stuff...