How long was Jesus's ministry?

I've always heard, over in over, in Church and elsewhere that Jesus's ministry on Earth was only three years. However, it doesn't seem that there is much supporting Biblical evidence for this, or at least direct evidence. Nowhere does it say an exact period of time the events in the gospel where taking place over. Is there evidence in the New Testament that we can reconstruct how long Jesus ministered for? If not then where does the three year figure come from?