Step 1: Parts!
You'll also need a li-ion 18650 battery.
Step 2: Conversion!
The battery holder is smaller than the 18650 but the springs inside the flashlight will compress enough to accept the 18650.
The three AAA's will be around 4.5V when they're fully charged but drop to around 3.5V shortly. The 18650 starts at 4.2V fully charged and drop to about the same voltage as the AAA's soon enough.