For a certain [man] named Demetrius, a silversmith, which made silver shrines for Diana, brought no small gain unto the craftsmen;
Note 1 at Ac 19:24: The name Demetrius means "belonging to Ceres" (Smith's Bible Dictionary). Ceres, also known as Demeter, was the goddess of agriculture. Demetrius was a silversmith who made small silver shrines of the temple of Diana or of the goddess herself. Because of Paul's preaching, many were turning from idolatry to the worship of the true God, and Demetrius and his fellow craftsmen were feeling the effects in their pocketbooks. This caused Demetrius to call together the craftsmen of like occupation. Although their true concern was money (1Ti 6:10), they turned this into a religious issue and incited a riot. This probably hastened Paul's departure from Ephesus (Ac 20:1).
It is very unlikely that this Demetrius and the one spoken of in 3Joh 12 are the same man.