Final answer:
The 3-word phrase for being happy that can be found in "Hey Diddle Diddle" is Happy as clam. This phrase implies that someone is extremely happy or content.
Step-by-step explanation:
The 3-word phrase for being happy that can be found in "Hey Diddle Diddle" is Happy as clam. This phrase implies that someone is extremely happy or content. It is often used to describe a person who is completely satisfied with their current situation.