These grow to a height of 1.5 metres, so it would have to be a very large container indeed! I would advise one very large pot per plant.

Depends how large the pot is - they do get 1.5 metres after five years in the ground, and probably only in very warm, sheltered areas, but they won't make that size in pots. And actually, I'm willing to bet they're half hardy anyway, so will need moving to a greenhouse or similar for winter. Thomson and Morgan's version is listed as half hardy, and the one you've bought has no hardiness ratings other than saying 'will need winter protection', which is sales talk for half hardy, mostly.

Given you'll need to get them under shelter for winter, using a large pot might not be a good idea if you can't lift and move it easily when the time comes. Hard to say whether three will fit for this year without knowing the depth and height of the pot you've currently got.