Difference of three input integers

Python 3, 21 bytes

lambda*l:sum(l)/2in l

If two numbers add to the other, the sum of all three will be double that other number, so half the sum will be an element of the list. Python 3 is needed to avoid floor-division, unless the numbers are given like 3.0 rather than 3.

Jelly, 5 3 bytes

Thanks to @Sp3000 for saving two bytes!

Code, uses quite the same algorithm as @xnor's great answer:

SfḤ

Explanation:

S     # Sum of the argument list
  Ḥ   # Double the list
 f    # Filter, remove everything that isn't equal to the sum of the list

This gives [] as falsy, and anything else as truthy.

Try it online!

ES6, 31 bytes

(a,b,c)=>a+b==c|b+c==a|c+a==b

Add 5 bytes if you need to name the function diff.

Edit: Saved 2 bytes thanks to @Alex L.