No Graves on Boot Hill (1968)

Original title: Tre Croci Per Non Morire

Three men meet in the same prison and in their cell they find a Mexican boy condemned to death for murder. One night the three convicts are mysteriously released by a priest and the father of the Mexican boy who promises them a reward of $30,000 if they can find the real murderer within ten days. Certain that the Mexican boy is not guilty, Reno and the others organize a plan which leads to the unmasking of the real killers.