Three triangular numbers
05AB1E, 10 bytes

Code:

ÝηO3ãʒOQ}¬


Explanation:

Ý             # Compute the range [0 .. input]
 η            # Get the prefixes
  O           # Sum each prefix to get the triangle numbers
   3ã         # Cartesian repeat 3 times
     ʒ  }     # Keep elements that
      OQ      #   have the same sum as the input
         ¬    # Retrieve the first element


Uses the 05AB1E encoding. Try it online!