I had Oracle 8.x client installed and accessing Oracle 7 and 8 db's. Then installed 10g client (and still have 8 client installed). I cannot access all 3 db's. I have to toggle the Oracle Home preference in order to switch between the version 7/8 db's and 10g. Is there a way to configure such that I can access all 3?

You will have to use the Oracle Net Configuration Assistant to set up a "local service name" for each database that you want to access. You will have to use the assistant from the correct home of course.