What is the most likely cause of the low compression in Cylinder 3 if it shows 0 psi during a compression test?

When Cylinder 3 shows 0 psi during a compression test, the most likely cause is a leaking head gasket. A head gasket is designed to seal the combustion chamber and maintain pressure within the cylinders. If the head gasket is compromised, it can lead to a direct path for air-fuel mixture or combustion gases to escape from the cylinder, resulting in a significant loss of compression. This scenario would explain the zero psi reading in Cylinder 3.

Leaking head gaskets can also lead to issues like coolant mixing with oil or external coolant leaks, but the immediate impact on compression is crucial for this diagnosis. While faulty components such as a timing belt, piston ring damage, or a leaking intake valve could potentially contribute to low compression, they often wouldn't result in a complete loss of compression as observed with a head gasket failure. Each of those scenarios typically yields some measure of compression, rather than the absolute 0 psi indicated in this case.