Is it possible to have a 3x9 setup - Triple 42T ( 42-32-24 ) x 9s ( 11-13-16-20-24-28-32-36-42T )? The main question is, what kind of 9 speed derailleur is compatible and how long chain need to be for this kind of setup? Thank you
If you had 42-32-22 in front and 11-42 in back, the derailleur would need 51t of capacity. A bit of casual searching revealed that the highest wrap capacity I could find on a newish derailleur is 47t, which would limit you to a 37t big sprocket in back.
Chain length isn't really a problem--you can find chains long enough for your purposes.
22:42 gearing is a ratio of 0.52 or 13.6 gear-inches on 26" wheels. That will be around 5 km/h or 3 mph in bottom gear at 90 RPM on the crank. You may not be able to balance at such slow speeds and high cadences.
Are you handy with tools? If you're capable of modifying a rear derailleur and fabricating a longer cage that may be one way to give more capacity. Downside is the chain gets closer to the ground and will get dirtier, and risks striking rocks more.
As for chain length - if one's not enough you'll need two packs of identical chain and join them. I find it better to buy three, so I have enough chain for the next replacement without having to source one more identical chain - this helps with tolerances.