The term Fratmogs actually means to build all three items (Atmas, Warmogs, Frozen Mallet).
Fratmas, by contrast, means to build Frozen Mallet and Atmas
and Atmogs, means to build Warmog's and Atmas.
The order of the build depends on the game.
However, usually, if I'm building a bruiser, I will build a health item first (Warmog's or Frozen), and then follow that up with Atma's, and then finish of the trio with whichever health is remaining.
What health item I pick depends on a number of factors, but it kind of boils down to one of 2 things:
- Do I need a lot more health and regen (Warmog's)
- Do I need more CC (Frozen Mallet)