Access panel for valves is behind the intake/exhaust manifold. The manifold has to come off to gain access. If the cyl had good compression then the valve setting isn't a issue. What type of tune up parts did he install- good quality or cheap? I've found champion spark plugs aren't well suited for lifts- A/C or autolite work well.

If you suspect cyl #3 is faulty- switch the spark plugs between cyl's 1 & 3 & try test again. If #3 is still suspect switch plug wires between 1 & 3. If after this #3 is still suspect remove all plugs & take compression on all cyl's while engine is warm & has no spark plugs installed.

If compression check is normal- manually turn engine over while observing valves thru spark plug hole- maybe cam is bad.