All good suggestions above. You can do this, it really is quite simple.
Three things are required to run well -
- fuel (right air/gas mixture),
- compression
- ignition (at the right time).
Determine which one of these three is missing. You have already verified spark and compression, right? Then the fuel mixture is likely not right. When you clean the carbs, be sure to check air flow through each circuit. Make sure it is similar on all three carbs. Make sure the starters (most folks erroneously call them chokes) are all seating right. The significant change you experience after the last carb cleaning suggest that you touched at least part of the problem.
Other things that affect getting the right amount of fuel into the cylinder include (as mentioned above) . . .
- reed valves not closing (easy to check with carbs off)
- exhaust passages blocked (pull the pipe and manifold to check)
- fuel pump pressure (check with a mighty-vac)
- fuel supply clogged (check carefully with air, check in tank filter/pickup)
- fuel tank low on fuel (not likely as it should affect all three cyls)