There can be much more to adjusting from a two to a three bedroom home. There may be other regulation involved that could be impacted based on regulations and codes......
If Zillow has it as a 3 bedroom and trulia has it as a 2 bedroom there is obviously a conflict. My guess is that your listing agent accepted your stated information without verifying the records while trulia based their information on what was presented in the tax records.
Either way, one must be very careful, making certain that the information that is presented is accurate.
Hope this helps.