If those are pole lengths rather than reach lengths, 24 ft should do most 3 storey places. However, there are always properties where the ground falls away a bit or you have to stand back and reach over things thus reducing the amount of vertical length available to you. 24 ft probably won't do all jobs like that. Apart from that, some jobs (particularly older properties) have taller ceiling heights so you might run into trouble on thse if you skimp on pole height. Can you drop a section easily with these poles? If so I would suggest going for the longest available and only put on the extra section(s) when needed. Maybe have some spare ends to protect the pole base in this situation? If sections aren't easily dropped, ultimately you may be better off buying two or three poles as by only buying one, you may end up dropping short on some windows or if doing it the other way, may be carrying excess weight when working. It really does depend on what sort of work you currently have, what sort of work you are likely to have in the near future, whether the sections come off easily, and how you might feel about using a much heavier pole than is needed for most of your work.
Rarely a straight answer because most things depend on individual situations.