No informality in any proceeding or informality in the conduct of any election, not substantially affecting adversely the legal rights of any citizen, shall be held to invalidate the incorporation of any district, and any proceedings, wherein the validity of such incorporation is denied, shall be commenced within three (3) months from the date of the certificate of incorporation, otherwise said incorporation and the legal existence of said district and all proceedings in respect thereto shall be held to be valid and in every respect legal and incontestable.