This is only MY opinion, & somebody correct me if I'm wrong, but you need to go back to your fish store, slap that guy clean in the mouth, suggest that he read or quit, and then leave, here's why...
Your tank didn't cycle yet, which means that the anticipated ammonia spike can kill those fish you put in there. Exception to this rule would be that your LS is fully cured and the trip home from the store is quite short. Now if that is the case you still have the matter of adding 3 fish at once without quarantine. Right now the qt isn't as much concern as the ammonia. If you are going to keep those fish in that tank, TEST your water frequently and be prepared to do many water changes. However, about your buffering question... Most people can do water changes to correct their ph. Some people drip Kalkwasser. And some add regular buffer, which the instructions to add should be on the back. Just be sure to test, when I added regular buffer, my dkh went to 11.0, while my calcium stayed at 300. (I don't know if that was bad for a FOWLR) but the dkh kept going up without raising my ph & I would have kept on adding until who knows what would happen if it weren't for this sight!!