Adam Clarke on Acts 20:31

Therefore watch, and remember - The only way to abide in the truth is to watch against evil, and for good; and to keep in mind the heavenly doctrines originally received. Unwatchfulness and forgetfulness are two grand inlets to apostasy.

By the space of three years - Τριετιαν. The Greek word here does not necessarily mean three whole years: it may be months more or less. In Act 19:8, Act 19:10, we have an account of his spending two years and three months among them; probably this is all that is intended. One MS., perceiving that the time of three years was not completed, inserts διετιαν, the space of two years.