if the average temperature of the Universe is 3 degree k what total mass does that represent floating around?

Answer 1

The average temperature has nothing to do with total mass. It's derived from the wavelength of the remnant cosmic background radiation from what we call the big bang using what's called a black body radiation curve. You could cool a Big Mac to the correct temperature and measure the same frequencies. Check here --

http://en.wikipedia.org/wiki/COBE

Answer 2

Eugene is right. 1 gram or 1,000 trillion grams could all be 3 degrees kelvin.