I have a very similar setup to yours, i.e. multiple machines, one ISP, but four mail accounts (separate identities, too, in order to separate newsletters, personal mail, a throwaway "junk" account to register with on sites that require it, and development environment [Delphi, etc.] news). I have found the solution proposed earlier to be perfectly adequate. I have all three of the machines on my home LAN set up w/OE and the four identities. All have "leave on server" set to two days.
To prevent needless duplication, I download my development-related mail (heaviest volume) to my primary development system. Personal mail I usually go ahead and check on all three to ensure that I have a copy in case of a crash. Newsletters vary, depending on type and content. Whenever I make an online purchase, I d/l that on all three to be sure I've got a copy of my registration data and any necessary activation codes.
I've found this to be a satisfactory way to maintain duplication when I want it, and avoid it when I don't. True, it's a bit of a kludge, but what the hey, it's simple and it works ;-). In a short time of using this method, you will work out your own pattern for how you want mail shared or not between the machines. I've been unsuccessful in figuring out any other way to accomplish this end, but in short order I found that it suited my style perfectly well. YMMV . . .
Cheers,
Scott