It reads like your cam gear is off, 120psi is very very low for a 3.0l, it is rated at 9.3:1 ratio.
My 4.0l hits 165psi average with over 300,000 on the rings, and it is only 9.0:1 ratio
If possible test another vehicles compression(one or two cylinder) with that gauge then retest the 3.0l.
Even a 1 cylinder lawn mower will be 90psi.
Or remove the valve cover on #1 cylinder side, and all the spark plugs.
Turn crank to line up TDC mark with 0
A 4 stroke engine has:
intake stroke, piston is going down and intake valve is open
compression stroke, intake valve is closed and piston is going up
TDC mark hits 0, spark plug fires
Power stroke, both valves closed, expanding gases are forcing piston down
Exhaust stroke, exhaust valve opens as piston comes up forcing burnt gases out the exhaust.
TDC mark on 0 again, there are TWO TDC for #1 on a 4 stroke engine.
And this is where I would check valve timing, the second TDC between exhaust stroke and intake stroke, the #1 exhaust valve should be closing just before TDC mark then be closed and the intake valve will be opening just past the TDC mark.