A 21-year-old Pico Rivera man who refused to buy beer for three teen-agers was shot by one of them as he stood outside a Whittier convenience store. Police said Paul J. Delgado was struck in the lower right side by a small caliber bullet that traveled across his back, narrowly missing the spinal cord, and stopped on his left side. Doctors at Presbyterian Intercommunity Hospital, where Delgado was in stable condition, chose not to remove the bullet because it was not in a vital area, police said. Delgado was approached by the youths after they were refused service at a Stop-N-Go Market on Beverly Boulevard. When Delgado refused to buy beer for them, police said, one of the youths drew a semi-automatic pistol and fired at him.