Do the hole patterns for SNS, NNN and 3-pin (including Back Country variants) cross-country ski bindings match up at all, or do you always need to drill new holes in the skis when installing a different binding system?
The different binding systems are of course primarily designed to provide a standard interface between the binding and the boot, not necessarily to provide a standard way of mounting the binding on the ski.
However that said I believe that the drill-pattern for SNS and NNN are generally speaking compatible, a situation that has some backing from the fact that I could find someone selling a universal drill jig for "Soloman and NNN bindings". I could not find something that ties in the older 3-pin system, but I suspect (but have no evidence for) that the mounting on the newer systems probably reflect the standard pattern of mounting the on the 3-pin system.
You should also be aware that in 2005 the NIS (Nordic Integrated System) was introduced, that uses a rail bonded to the ski that is compatible with both NNN and SNS bindings - so no drilling required with compatible skis.