Group 3's are probably the best choice right?

Not necessariy :-) You DO want one that will be fully hardy, but as a majority of the hybrids are listed to zone 3, you are not too limited.......the "rule of thumb" for successful long term container culture for any plant is one that is two full zones lower with respect to hardiness.

I'd look one of the Evison clematis that have been bred specifically for container culture - often labeled as "patio" clematis or sold under the Boulevard clematis tag. These tend to be more compact vines and extremely floriferous.

I also like 'Harlow Carr' for containers. This one is a blooming machine!

Here is a link that might be useful: Evison's patio clematis

Well, I've settled on Madame Julia Correvon for my whiskey barrel. She's supposedly got the hardiness, the vigor (important in my northern zones), and isn't yet another purple/lilac (got plenty of those already :) ). I think she'll do nicely, but we'll see. I bought 'er at Joy Creek, on of my favorite places.

I think MJC will be hardier for you than the Patio Collection. MJC has much more viticella blood in it,the Patio Collection has more patens/lanuginosa. In my zone the former are less affected by winters and come up sooner than the latter.

If you are filling a wine barrel, why not add at least one more? This will be spilling out, I suppose, so the Harlow Carr suggestion is a good one - long bloom period and dark coloring, plus much more attractive seedheads than a viticella. Or any of the other non-purple viticella/integrifolia/diversifolia groups that would contrast:

Emilia Plater
Hendryetta
Mrs. T. Lundell
Jan Fopma (ok, purple, but shiny purple)
Juuli
Arabella

You don't think that would be too many in the pot? I don't want to take away the nutrients from one or the other.

Thanks, I also thought a viticella (which I've heard sooo many great things about that group) would be good. With our short growing period and cold winters here, it made sense to me. Now, some viticella's (most I think) are way too large for a container, but Joy Creek as well as the American Clematis Society says that MJC is suitable for a container. And, you can't beat the hardiness.

Plants grown in a container - any plants, not just clems - will need frequent fertilization. Container plants need more frequent watering than do those planted inground and that frequent watering leaches out nutrients that need to be replaced. Plus, container grown plants do not have the benefit of the access to all those nutrients existing in garden soil and container potting soils tend not to have high levels of nutrients anyway.

So.....since you are going to have provide additional fertilization on a routine schedule anyway, it really doesn't make any diff if there is one vine in the barrel container or three!! Pick ones that are compatible and have similar growth habits so one doesn't overwhelm the others. btw, the patio collection all tend to be very long bloomers - I'd rate most of them as having a significantly longer bloom season then either the traditional group 2 or 3 vines.