You can map the slider to any variable or “axis” type of control you want. Access the Controls menu and double check that the slider is mapped to your throttle.
The Throttle in any aircraft works just like the accelerator pedal in your automobile. It controls the amount of air - and with that variable the amount of fuel - your engine is getting.
Manifold pressure is typically monitored in an aircraft equipped with a Constant Speed Propeller. The throttle’s function in those aircraft does not change, however, the way you monitor the power is different. RPM is held constant by a governor that controls the pitch (angle) of the propellor blades. That variable is controlled by the prop control (many times - but not always - colored blue) and is adjusted in concert with the throttle.
The DA40NG you mention is equipped with a constant speed propeller, so rpm should be constant in that aircraft in most circumstances. Like the Cirrus SR-22, the DA40 has a “single lever” engine control that eliminates the traditional “prop control” lever. The VL-3 you mention looks to have more conventional manual control of the propeller, using a traditional “blue lever” to control the pitch of the prop. The Robin DR-400 you mention has a fixed pitch propeller, so will not have a prop control and because of that, you’ll see engine (and propeller) rpm drop as you reduce throttle.
All three aircraft are very different in the way they handle power management, so having controls set up as a “one size fits all” solution will present a problem. Perhaps you could set up profiles for each of them in the Controls menu?
Hope this helps… cheers!