DO NOT change to a 4 wire, you will be causing more problems as Kevin says. The cause of your problem needs to be determined, not guessed at or start replacing the whole thing. It already works. Several possible reasons for your issuue that take a troubleshooting procedure.
Some thoughts are.
1. Low/failing battery. What is voltage of battery at rest, motor off?
2. Generator/voltage regulator. What is output of generator at idle and high idle?
3. Loose/corroded ground somewhere. Clean all battery cables, posts and any ground straps connections you find.
4. Bulb failing. Look for dim or jittery bulbs.....or dead bulb.
5. The switch. Any work done lately on any electrical whatsoever?
6. Try to simulate the problem at home so you can walk arounnd truck and inspect each bulb's function at various motor speeds and light loads. Don't dwell on wire colors at this time.
7. Report your observations and if you have done the above tasks/tests. You will need a multimeter and nice to have an "ice pick" style tester.
