It sounds like you're using the Samsung Keypad with "Portrait keypad type" set to 3x4 Keypad, and "XT9" switched off?
For starters, your device has a lot of different keyboard types pre-installed, if you switch to a different keyboard or mode, you can just press "..." with 3 presses on the "." button.
Before I go into your other keyboard options, you can teach the phone the "..." sequence as a new learned "word". Go into Settings -> Locale and text -> Samsung keypad, you should now see an option XT9 make sure that this is ticked, and then you can go into XT9 advanced settings and scroll down until you see XT9 my words press Add word and type in the three dots as if they were a word and save.
Now when you're typing you should be able to do it as
dot (wait) dot dot which is slightly faster.
Alternatively change your keyboard mode, go into Settings -> Locale and text -> Samsung keypad, Portrait keypad types one of your options here is Qwerty keypad this keypad looks a lot like a normal computer keyboard, and you can just press
dot dot dot without any need for pauses.
Finally you should also have the Swype keyboard pre-installed on a Galaxy Tab, to turn this on go into Settings -> Locale and text -> Select Input Method and pick Swype. This looks a lot like the Samsung Qwerty keyboard, and you can use it the same way to do
dot dot dot, it also a totally different and fast (once you've had some practice) way to enter words by swiping your finger across the keyboard. Swype should pop up a tutorial to show you how to do this when you first switch it on.