#3 (I will tell her that I love her) is the only correct sentence. 'Say' focuses on the words someone said and 'tell' focuses more on the content or message of what someone said. So if you wish to communicate your heartfelt feelings to someone ('love'), the correct verb is 'tell.'

2 and 3 are right.

the 1. is incorrect. others are correct.

"say something to somebody" but "tell somebody" - are the language models