That is a very unique situation and it really depends on whether you want to use Poser on all 3 of those. I'm not even sure how many computers you may have Poser running. This is just what I think, others may do it differently.

I would create a download folder on an external hard drive but would NOT put the installs on it in this case. I would put the raw zips on an external drive that you can take with you and install the content on the different computers individually the same way I showed you. The reason being, if you have the content installed on external drive and you transfer that back and forth, you need to, every time you decouple and recouple so to speak, tell Poser you got those folders of Library back. I just would prefer content existing on each computer  individually and just add the new content every time you get a new product purchased. I never used Poser on more than 1 dedicated computer, so I can't really give you a real life answer; I let others do that who experienced it.