When you hear the valves start to float, shift. :D
But, more seriously, you have a small engine without a ton of power. So, you'll want to shift to keep the engine in it's power range. Shift too low and you'll bog down. Rev too high and of course you can damage the engine. But the gear spreads on that 3 speed are pretty wide, so, you'll have to shift significantly higher than with a transmission that has more gears if you want to keep the engine in its power range when it drops into the next gear.
It will also depend on your rear axle gearing and tire height, which you haven't mentioned.
@AJ/FormS is the gear ratio guru, maybe he can get you some more exact info based on your set up.