Is it possible to put an after market 3 postion safety on my 375 CZ?

If your talking about the CZ 550 I haven't found any aftermarket safety for these. You might do a search for gunsmiths that work on Mauser actions as that's what the CZ is a copy of. One of those may be able to build you one simular to the Winchester model 70 safety as these were available aftermarket for the Mauser actions.