Why should it be???
as far as I know people that want to highlight a line in MOST windows editors either use the shortcut... sublime is ctrl+L, click on the "gutter" (numbers line), or do shift+end/home, or last use the mouse..., double click by standard in MOST windows editors select the current word under the cursor... try it in notepad... triple click... nothing?
Now, I am not picking up on u or your idea, I think is good to have choices but the "why not!?" attitude is not good
Bad news is that I don't think you can even do a plugin for it at least not with sublime api, you can try autohotkey, I think it could be possible with a PostMessage to send the command to highlight the line with triple click...