All substituted nomination certificates for special elections may be objected to, as provided in section 977 of this act, except that objections to substituted nomination certificates must in any case be filed within three (3) days after the filing of the substituted nomination certificate: Provided, however, That no objections as to form and conformity to law shall be received after the day on which the printing of ballots is started.
25 P.S. § 2785