The probability that the red cube WON'T be picked on the first draw is 2/3. This is because there is a 1/3 probability of picking a green cube, added to a 1/3 probability of picking a blue cube.
The probability that the red cube WON'T be picked on the second draw is 1/2. This is because one cube has already been picked, so there are two remaining. You can only pick one of the two.
Let's say you picked the blue one first. You have a 2/3 chance of doing that, and now you're left with 2. Now, to pick the green one, you have a 1/2 chance. Multiply that and you see that you have a 1/3 chance of picking it last.
