#1 #2 #3 low compression in L6 250

[c10hevy]

In my 66 GMC I did a compression test today because it would stall out many times when starting in 1st gear. The results:
#1: 90psi
#2: 85 psi
#3: 75 psi
#4: 120psi
#5: 120psi
#6: 120psi

I was thinking the rings are leaking in the first 3 cylinders but after doing some searching ive found that it could possibly be from stuck valves or possibly even flattening of the camshaft.

Any help would be greatly appreciated.