Assuming an American Roulette wheel with a Zero and a Double-Zero:
If you said, "The next 3 spins in a row are going to be 33", then the chance is 1 out of 38 cubed (as someone said), or 1 out of 54,872.
If you hit 33 once and then said, "Wow. That's great I won. Now I'd like to hit 33 TWO more times in a row!", now the formula would be different. Since you've already hit the 33 once, now your chances of hitting it 2 more times in a row are 38 squared, or 1 out of 1,444.
If you hit 33 once, and then hit it a 2nd time in a row and THEN say, "Yeah! I wonder if I can hit it a THIRD time in a row!", it's a lot easier at that time than you would think. Once you've already hit it 2 times in a row, the chances that you'll hit it a third time are simply 1 out of 38.
You aren't going to see this while you play very often, and it's especially nice when you are on the winner, but if you walked around from casino to casino, from table to table, in Vegas or Atlantic City or anywhere else where they had many casinos, you would be able to see this happen at least once per day.