In the “D,” “E” and “F” Districts, if the property is not used for dwelling purposes, no side yard is required unless the lot abuts upon the side of a lot on an “A,” “B” or “C” District in which case a side yard of not less than five feet (5') shall be provided on the side abutting said “A,” “B” or “C” Districts. In any case where a side yard, is provided but is not required by this article, such side yard shall be not less than three feet (3') in width.