Prove binomial from Multinomial

Suppose that (X1,X2,X3) ~ Multinomial(n,Theta1,Theta2,Theta3). Propve by summing the joint probability function that X1 ~ Binomial(n,Theta1).

I struggle with proofs and am having a big challenge with:
1. Determining a starting point.
2. Determining what a "finished" product even looks like. What is the result of the work supposed to be?

Thank you.


Ambassador to the humans
The starting point would probably be to write out the joint pmf for the multinomial distribution and then figure out what the appropriate sum will look like. What have you tried so far?