Clematis: Ordered these and what size pot should I use, 2 by pirl

pirl wrote:
Gourd - About two years ago I moved three clematises in front of a long established Jackmanii with the vision that it would be a mass of creamy white (Guernsey Cream), pale lavender (Proteus), very pale lavender (Belle of Woking). Instead I have a mess because the Jackmanii came forward this year but in all previous years went to the rear to cover the compost bins. I did cut it back on May 5th this year and had cut it to the ground at the start of the season. I'll have to move all but the Jackmanii this fall. That is why I'd be very hesitant to plant three in a huge pot.

1. How it looks as of yesterday. The one trellis has been swallowed up by the Jackmanii.

2. My original vision of the area - ha ha ha.