Wait, that was dumb. Of course it isn't. The first instance of a game would simply lock all the resources for itself.
Shared reads are possible though, so in theory you should be able to - funny enough, in this case mods are probably the worst offender* with them opening log files in exclusive write mode and possibly not handling failure correctly so they would possibly malfunction when loading on the second game instance.* Elaborate SilentPatch™ development technology is free from such mistakes!
Edit: What's the resource usage when all the three games are launched? Any noticeable lags in mid-range PC setups? Did you make any tweaks to balance the resource use in-between the games?
Nope, I didn't test it extensively - I only ensured they don't conflict with each other, since they had no reason to. Maybe it runs like a dog on slower PCs, maybe one game steals resources more - I don't know, I don't care either.