Sorry for awakening an old thread but was trying to fix a similar issue myself.
Vec3d normal = v3.subtract(v2).crossProduct(v1.subtract(v2)).normalize();
Just leaving this here in case it helps someone searching for answers.
Made a Forge Json generator from texture path if useful for anyone.
Link to script:
https://github.com/MoreThanHidden/MTHPowershellScripts/blob/master/BatchCreateForgeJson.ps1
Example:
If your not familiar with powershell you may need to run via:
PowerShell.exe -ExecutionPolicy Bypass -File .BatchCreateForgeJson.ps1