Cards with Digits

Rachel has three cards with different digits, all of which are greater than 0. Rachel formed all possible three-digit numbers from these cards and calculated their sum.
Prove that the sum is divisible by 3

Difficulty level (1 very easy - 10 very hard): 2

Topics:
Number Theory -> Modular Arithmetic / Remainder Arithmetic -> Divisibility Rules -> Divisibility Rules by 3 and 9 Logic -> Reasoning / Logic Proof and Example
Sources:
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