Your gauge lights are powered by the gray wire which goes to terminal 8 on the cluster plug on the original harness, and they are grounded to the cluster by two small screws which fasten the copper strip to the cluster metal. Your harness wires are not colored the same as the factory wires which will make it more difficult to follow the wiring.
This diagram shows the circuit and it should help you figure out where the problem is.
The picture does not show the copper strips but it has colors in place to help trace the path of the circuits.
Notice the number 8 terminal on the cluster plug with the gray colored line going to the right hand light and to the other lights on the cluster. I think you might have a tiny fracture in this copper strip which would account for the symptoms you have.
Ray beat me to it.
The orange line is the ground line for all the bulbs, and you say that all but the three instrument lights are working. Plus you have checked the grounds to the cluster and added some.
You can trouble shoot the cluster if you remove it and apply 12 volts to the no.8 terminal and ground the two small screws. The lights should come on.
If the same three fail to light then apply power to the first light past the fuel gauge . Then the next in line. Be careful not to short the power to the ground strips (orange).
Or you can do a continuity check on the power line for the lights from terminal 8 to the last power point on the light farthest from the number 8 terminal to see if there is a break in the strip.