They are all correct. The difference is how strongly they show reality vs. possibility. But the difference is really only technical. Usage is exactly the same. 1. If I were to tell the truth, you wouldn't believe me anyway. 2. If I tell the truth, you won't believe me. 3. If I told the truth, you wouldn't believe me.

二番目は将来のことを予想します。 一番目と三番目は過去のことを仮定しますが。一番目はちょっと惜しい気持ちがあると気がします。