The OED is certainly an impressive source.
But, I would like to point out that it IS a British source, and in American terminology I have never seen "come" considered to be a preposition -- not that I have looked deeply.
Probably the OED gives a history and an etymology for that use of "come" -- as well as contexts.
"From" and "by" are quite different, in that they are never verbs. The prepositions "down" and "off" can sometimes be verbs -- but transitive ones.
- For Teachers