You can tell a three year old anything you want, and get any reaction you desire. It's all in the presentation.
"Guess what, Boy Terror? You get a special treat!"
"Yay!! What is it what is it what is it?"
"You get to go on a fun trip to.....(drum roll please) the DENTIST!"
"Woooo! What's a dentist?"
"He's a special teeth doctor. And guess what he has for you?"
"Fun stuff to clean your teeth, and a really cool chair just for YOU to sit in, and a neat light over top!"
"Wow! Why am I getting to go?"
"Dentist is going to do a ROOT CANAL! Doesn't that sound interesting?"
"I think we'd better go mark this special day on the calender."
And off he goes to gloat to his sister that he gets to have a root canal, at the dentist.
Girl Terror knows this trick, unfortunately. She used it on her brother last week when she had to stay home sick, while he went to daycare. She told him very seriously that she would be staying home with Daddy, watching videos and snuggling and playing, while he had to go off to day care All ALONE. She kept it up until she made him cry, and then she was satisfied.
I came into the room, and told him "You get to go to daycare! Just you, not Aidan. You get to play with all the kids! You can tell them your sister is soooo sick, and she can't do any of the fun things. YOU can do crafts, and play with the kids, and have a yummy snack! Don't forget your important job of telling Wendi that Aidan is sick!"
Off he went, thrilled to pieces. It's all in the presentation.