from an older thread (STEP loader antlr v4 based · Issue #7125 · mrdoob/three.js · GitHub):
STEP is complex, and in most cases will need to be processed by a CAD kernel to get to a format that three.js can currently load. The reason is that STEP contains several representations for geometric shapes, the most common being Advanced BREP (often with NURBS and trimming). There is an open source project that generates a low-level API for STEP data (www.stepcode.org) that is used by BRL-CAD and OpenVSP. OpenCasCade has its own STEP translation capability, which is used by FreeCAD (and others) for STEP and IfcOpenShell for IFC. Clara.io can import both STEP and IFC, but like many other free tools (including open source such as FreeCAD and OpenCasCADE) lacks modern STEP capabilities like associative 3D text, supplemental geometry, model styling and organization, tessellation, user defined attributes, PMI, etc. (www.cax-if.org). The NIST STEP File Analyzer can convert most modern STEP capabilities to VRML/X3D, and is free but not open source. So unless three.js develops capabilities to cover more of the STEP geometric representations, STEP and IFC data will need to be processed by something into a form that three.js can load (like three.js json, STL, VRML/X3D, COLLADA, etc.)
not sure if that is still accurate
will it work together with threejs?
yes. but again, it’s not released. you’d have to request access.