Here the 1837 edition omitted the modal verb shall. This deletion could be an error by the 1837 typesetter; the change was not marked by Joseph Smith in the printer’s manuscript. Moreover, in the original (and current) text, there are 16 other examples where shall appears in except clauses, including five with the exact phraseology as originally in Jacob 3:3 (“except ye shall repent”): Helaman 7:22, Helaman 7:23, Helaman 9:22, Helaman 15:1, and Mormon 5:22. Elsewhere there are 18 occurrences of the phraseology “except ye repent” (that is, without the shall, including one nearby in Jacob 3:4 (which may have had some influence on the 1837 loss of the shall in the preceding verse). Interestingly, in Helaman 9:22 Oliver Cowdery initially wrote the phrase “except ye shall repent” without the shall, but he caught his error almost immediately and supralinearly inserted the shall (without any change in the level of ink flow). Obviously, the shall is definitely possible in “except ye (shall) repent”.
Summary: Restore in Jacob 3:3 the modal verb shall to the except-clause (“except ye shall repent”).