The answer is 6.
Think about the different marbles as marble '1', marble '2', and marble '3'.
If you fix the marble '1' to be the first pick, you're going to have two distinct possibilities.
_1_ _2_ _3_ and _1_ _3_ _2_
If you fix marble '2' as the first the same idea applies. If you multiply the amount of possibilities for each fixated marble by the total amount of marbles, you're going to get 3x2 = 6.