Which is correct try saying three tall trees or to say trees?

Both are grammatically correct, but they have different meanings:

* "Try saying three tall trees" is a command or suggestion to repeat the phrase "three tall trees." It's focusing on the act of saying the words.

* "Try to say 'trees'" is a suggestion to attempt to pronounce the word "trees" (perhaps it's a difficult word for someone to say). It's focusing on the pronunciation of the word.

So, the correct choice depends on what you want to communicate!