Oliver Cowdery initially wrote “telling them of the awful consequences of them”, but then he crossed out the of. Elsewhere in the text, the verb tell typically does not have the preposition of, but there are eight instances of “tell (someone) of something”:
Note that half of these examples involve the plural noun things. On the other hand, there are 23 occurrences of the verb tell where the complement is things but without the of, including one example without the of for which there is a matching example with the of:
In each case we therefore let the earliest textual sources determine whether the verb tell takes the of or not. Here in Jacob 3:12, the crossout of the of looks virtually immediate, which argues that in this instance the original manuscript lacked the of.
Summary: Maintain in Jacob 3:12 the current reading without the of that is sometimes associated with the verb tell: “telling them the awful consequences of them”.