saying fat is not a proper way of saying this...it is offensive/rude. Replace fat with overweight.
the first sentence is ok but not natural
the second sentence is perfect (with the revision I offered above)
The last should read "You wouldn't look good if you were overweight"
Honestly, I don't know if any of these sentences are welcomed/polite/appropriate. Use them with care, said to the wrong person could get you slapped HAHAHA!!