1. Just say "Pitstop, add fifteen litres" or "Pitstop, fill to twenty five litres".
2. Say "Pitstop, medium tyres". If you want to do it with a key press Add/Remove Actions and you'll find "Pitstop medium tyres" in the list.
3. Not sure, that used to work. The only thing I half remember is that rFactor doesn't always allow you to click down to 0? Can you do it by pressing keys in game? You may get clues by checking the "Log type - verbose" Property, that will print out details of what's happening when CC is operating the Pit Menu.
Pitstop management Help page