suppose you have to pay $1 to get a three digit combination at a carnival.
if your three digits are a winning combination you win $100, but they keep your $1.
if you don't get a winning combination they keep your $1.

a.if you have 10 C 3 what is the probability it's a winning combination?
what's the probability its not?
b.what is the mathmatical expectation for 1 pick?
c.what would they expect to make from a sale of 1000?

Couple of questions for clarification back to you:

Please confirm that the 3-digit combinations are random and independent - meaning you can have any combinaton from 000 to 999 (1000 possibilities), and if you play twice you may get the same combination both times.

What do you mean in (a) by "if you have 10 C 3?" Are you saying if you have 10 C 3 picks what is the probability of at least one winning combination?

In (c), are you asking what the carnival can expect to take in if 1000 people play? Again, this will depend on whether any two people could have the same combination, so you need to clarify that for us.