I'm going to plant it in a big chimney pot would that be okay the bottom is open
That would help a lot.
I would mix multi-purpose compost 50/50 with John Innes compost if it was me. The John Innes is much heavier as it is soil based and keeps its structure. Multi-purpose tends to degrade and go to dust over a year or so. It would also make the chimney pot a lot more stable and less likely to fall over.
I have a bag of John Innes no. 2 on hand most of the time for adding into compost mixes. There is not a lot of difference between no. 2 and no. 3 except the amount of feed in it, so No, 3 would be OK as well. Some recommend using John Innes alone for permanent containers, but it is a bit too heavy in my opinion. Plants seem to do best for me in a mix.