Where is the reference in bible that JESUS preached for three and Half years? Required bible reference that JESUS Preached for three and half years.
I agree that this is a tricky question and the answer depends on the tradition to which the answerer belongs.  Here is a simple, yet plausible scheme that not everyone will agree.

In the Gospel of John, who appears to be quite careful about his time elements, we have a series of Jewish feasts mentioned.  The most significant of these feasts for chronological purposes is the Passover feast which occurred about March/April in our modern calendar.


John 2:13, 23 - Passover feast a few months after Jesus' baptism
John 5:1 - unnamed feast but probably the Passover.
John 6:4 - Passover feast
John 7:2 - feast of booth (or tabernacles) 6 months after feast of Passover
John 10:22 - feast of dedication - a few months later
John 12:1 - Passover feast when Jesus was crucified


By this simple account Jesus ministry began several months before His first Passover during the 15th year of the reign of Tiberius Caesar (Luke 3:1) and ended at His crucifixion about 3½ years later.