Since such markers are likely to be flat / 2D - for as long as you stick to web / browser - using CSS2DRenderer (or CSS3DRenderer) may be the way to go (you can then use DOM elements - text, divs, img’s etc. to create the markers.) CSS2DObjects are easy to create, update, and they still have a position in 3D space.
If you really want it to be a Mesh, because life’s an adventure:
(1) It’s quite easy to create custom shapes using ShapeBufferGeometry. You can also solidify these shapes by extruding them.
(2) To create numbers you can use a TextBufferGeometry (this can also be extruded.) For an easier way to handle text you can also take a look at Troika-3d-text: library for SDF text rendering .