code: def forward(self, x1, x2, x3):
      return x1 + x2 + x3
  
file_path: tinygrad/test/extra/test_export_model.py
imports: import unittest
  from extra.export_model import export_model, EXPORT_SUPPORTED_DEVICE
  from tinygrad.tensor import Tensor, Device
  import json
label_desc: Use image node_id 1 for calling the MockMultiInputModel obj's underlying member method code with example usage: obj.forward(x1, x2, x3) and returns: unknown