You have two of three right:
- One position is the Drive, where the wheels are connected to the engine via the gearbox
- The middle is the Neutral, where there is no connection between the wheels and the engine
- And the third should be the Reverse, where the wheels are connected to the engine but with passing through a reverser instead of the gearbox.
This is the relevant part, the lever in question hooks into the gray transmission driving ring in the middle and connects the central axle either to the red or the blue gear:
If the reverse is indeed missing, you might have made an error during assembly. Finding and correcting such an error will most probably involve disassembling the whole bodywork and some of the undercarriage, then rebuilding everything back again. For this reason it is generally a sound advice to double-check the parts added in each step and periodically check the already built subcomponents to see if they work as expected.