code: def pdf4(self, x1, x2, x3):
      return multivariate_normal.pdf(
          [x1, x2, x3], [0, 0, 0], [[1, 0, 0], [0, 1, 0], [0, 0, 1]]
      )
  
imports: import unittest
  import numpy
  import numpy.testing
  from scipy.special import beta
  from scipy.stats import multivariate_normal
  from pgmpy.factors.continuous import ContinuousFactor
label_desc: Use image node_id 4 for calling the TestContinuousFactorMethods obj's underlying member method code with example usage: obj.pdf4(x1, x2, x3) and returns: multivariate_normal