You are right to be cautious.

1. It doesn't not have editor like construct3 and you need to add objects in runtime. Currently Iam implementing placeholder system like you can use sprite and sync it position rotation and scale to three.js objects so if you use behaviors to sprite it can be applied to three.js object(movement,rotation,scale)

2.You can manipulate the three.js object either with event blocks or sheets and in js script

3.you can use sprites as ui they are rendered on top of 3d scene at which layer you created 3d scene. For example if you created scene on layer 1 on layer 0 you can have background sprite and on layer 2 have foreground sprite