If you want to choose which Rhino version to use to open a .3dm file, right click the file instead of double-clicking it, and select the ‘Open with’ option.
If you always want by default to open R7 files with Rhino 7 and R8 files with Rhino 8 (except where you use the above override) you can use my RhinoFeeder utility, as found here: Open Rhino 8 files - open with problem - #5 by jeremy5.
The .ini file in that link will likely need updating if you want to cater for the current WIP as well.