I don't know Olympus well, but I do think that if I were in the same boat as you, with a third 35mm SLR camera system that I just sort of happened into, that I would: 1. get a 50mm f/1.4, even if it is redundant, simply because every 35mm SLR that I am going to shoot should have my most-used, and probably the most versatile, lens, and 2. After the 50, shy away from redundancy. I would seek out lenses that are unique to the system, or in focal lengths or speeds that I do not have in the other systems.
I do shoot Canon FD and Nikon F systems, and I have a Pentax Spotmatic that I got for free and a K1000 that I got for five bucks, just in case cheap and good lenses should come my way. All the systems have a fast normal lens. Other than that, there is little redundancy between them. I have 28, 55, and 200, and 300 for Canon, and 24, 35, and 135 for Nikon.
Oh yeah. I almost forgot EOS. I have two bodies (one digital and one film), and my only lens that I actually own is the 50mm f/1.2. Why is my EOS system so limited? Because it is EXPENSIVE STUFF (and I don't use it all that often any more).