I want to send a message to my boyfriend on the third day after breaking up.

Scold here, don't send it to him, such a man is not worth scolding.

As you said, he just uses you and doesn't care about your feelings at all, so it will be disgusting for love rat to talk to him any more.

Who didn't meet a few dregs when he was young?

Delete all his contact information and let this scum be physically removed from your world.

Soon you will meet someone who cherishes you.