I know there is the grouping method:

...

//after loading cube models
const group = new THREE.Group();
group.add( cube1 );
group.add( cube2 );
group.add( cube3 );

scene.add( group ); //This gives me 3 calls when rendering

but this doesn't affect performance, it's just to make things synthetically more clear.

I would like to merge cube1, cube2, & cube3 into a single object to reduce the number of calls in the scene

import { BufferGeometryUtils } from '../jsm/BufferGeometryUtils.js'; //gives error: BufferGeometryUtils.js doesn't export BufferGeometryUtils

var geometries = [];

geometries.push( cube1 );
geometries.push( cube2 );
geometries.push( cube3 );

const cubes = BufferGeometryUtils.mergeBufferGeometries( geometries );//doesn't work (outdated?)

scene.add( cubes );

I've followed some tutorials on BufferGeometryUtils, but they seem outdated since BufferGeometryUtils.js doesn't export a BufferGeometryUtils constructor