This mod is an addon for Tabula which lets you export your models in the entity json model format for ThreeCore. If you use the "threecore:default" type there is not much you need to take care of, but if you plan to make armor models with the "threecore:biped" type, there a few things you need to take care of:

Open the Tabula menu, and click on the green arrow with the small Minecraft Icon on it. Then import "ModelPlayer" (there are two for the steve/alex models).
This might look a bit messed up, so you can delete all parts you won't need for your models (example: if you only plan to add ears to the head, delete everything except the head part)
Now you can make your model parts, BUT you'll need to parent them to the existing parts.
When organising the texture map, make sure your model parts don't take the space of the model parts of the player model (including the ones you deleted earlier). Extending your texture size to something beyond 64x64 might help. You can however put your texture parts in that space if you disable the model part that is supposed to have their place on the texture map there. To do that, you'll need to manually write that into the exported json later on like this: "disabled": ["head", "head_overlay", "chest", "chest_overlay", "right_arm", "right_arm_overlay", "left_arm", "left_arm_overlay", "right_leg", "right_leg_overlay", "left_leg", "left_leg_overlay"]
And now you can export the json!

Tabula ThreeCore Exporter