I'm attempting to create a small 1st-person game using Three.js, but I'm having trouble with the lighting. Basically I want to simulate the sun and have it rotate around casting light on everything. I'm using THREE.DirectionalLight at the moment and it only lights up the one direction so sides of cubes remain black/dark.
Do I have to use multiple lights so everything is lit up? Or could I somehow reflect light off the ground/objects?
Yes, you'll have to use multiple lights to achieve this, faking the reflected light. Computing real reflected light isn't built in (and computationally very complex/expensive). You have a variety of options.
A second directional light that could always be in the opposite position and direction of your sun.
A hemisphere light that remains constant. Hemisphere lighting gets a sky color and a ground color and intensity and adds a nice extra bit of depth to your lighting.
// sky color ground color intensity
const hemiLight = new THREE.HemisphereLight( 0x0000ff, 0x00ff00, 0.6 );
Here's a working example https://threejs.org/examples/#webgl_lights_hemisphere
you could use any combination of different lights but be careful there's a performance trade-off.
It's also worth mentioning half-lambert shading which is enabled by setting the wrapAround property to true in your material. This gives a nicer falloff to black that results in less harsh lighting. More mid-tones and less blacks.