fill comma separated list

[papab]

I have a whole bunch of sequences of numbers, like this:
   5.1816,             
   -3.81,              
   4.8768,             
   5.1816,             
   -4.4196,            
   4.8768,             
   6.4008,             
   -4.4196,            
   5.48640,  
   6.4008,             
   -3.81,              
   5.486400;

I'd like to be able to make it look like this easily:
   5.1816, -3.81,  4.8768,             
   5.1816, -4.4196, 4.8768,             
   6.4008, -4.4196, 5.486400,  
   6.4008, -3.81,   5.48640

3 numbers per row.
Suggestions?  Some variation of fill-paragraph?