According to আইএমডিবি Penny has been in 142 episodes while Sheldon and Leonard have been in 141 each. What episode are they not in?

আইএমডিবি is incorrect. Leonard and Sheldon have been in every episode (as have Howard and Raj) but Penny has not been in two (4.05 and 4.06) due to Kaley's horseback riding accident.