Not true of Montana types, these are usually left to do their thing untrammeled, but can be pruned back after flowering, usually by end of May. Montana varieties do get very large, best not grow the group 3 clematis near those, it'll be buried and lost amongst the montanas. Group 3 refers to the pruning group, and yes, they should be cut down in midwinter, not later than 15th February. Bit late to do it, but you can still do it if it needs it now, otherwise, let it grow and make sure you do it this winter.