U choose what is the z best 4 U!

Congratulation for the multi offer!
You always choose what is the best.
Good thing is you have not started, so you saved there IT group's time, they do not need setup your PC.

I will send another mail say: I have to reject the offer.

It is very easy.
There is nothing to worry about.
In the future, if that Agent can sell you , he/she will.
If that company need you again, they will hire you.

Again, you choose what is the best 4 U!
Thanks!