I think the best and easiest way to do this without a lot extra steps like the token way is to use arrays. Plus its scalable for any amount of numbers/strings/etc.
Make an array and fill it with all your possible choices (in this case numbers) pick a random number from the size of the array and then get that number out of the array and store it however you want to store your 3 picked numbers. Then remove that number from the array.
Once its removed from the array it wont be able to be picked a 2nd time cause it no longer exists unless you added it more than once to the array