Three things to watch for Sunday in the San Francisco 49ers' third exhibition of the 2013 season, set for 8 p.m. ET at home against the Minnesota Vikings:1. Seneca's debut. Recently signed veteran quarterback Seneca Wallace is scheduled to make his 49ers debut as a candidate for the No. 2 job. Will he provide an attractive alternative to Colt McCoy, Scott Tolzien and rookie B.J. Daniels? Wallace has little time to learn the 49ers' offense. However, the system does overlap with the one Wallace, 33, ran for years while playing for Mike Holmgren and the Seattle Seahawks.