You can use the STLLoader class for this task. So you define an instance of the loader and use it to load a STL asset from a specific path. In the onLoad() callback, you get an instance of BufferGeometry representing the geometry data from the STL file.

const loader = new STLLoader();
loader.load( './models/stl/ascii/slotted_disk.stl', function ( geometry ) {

    const material = new THREE.MeshBasicMaterial();
    const mesh = new THREE.Mesh( geometry, material );

    scene.add( mesh );

} );

Keep in mind that the geometry is non-indexed. So the faces/triangles are defined by three consecutive vertices in the position buffer attribute.

Full example: https://threejs.org/examples/webgl_loader_stl