Replay the invitations "Writing Pre-Int"

1. Less Formal:
Hi David!
Thanks you so much for inviting me, but i cant. I have to meet with Rosita at the same time, so..I'm sorry!
Thank's again and keep in touch!
Have fun!
Kate.
2. More Formal:
Dear Sarah,
Thank you for the invitation. I will come for sure. I'm really sure that will be a great night.
See you on saturday
Greetings
Matthew.
3. Text:
Yes, I am free! Just let me know time and place C U there..
BEA!