left mouse button always double/triple click

I would say it depends on how old it is, but mice do this frequently. In short take it apart and clean it, that should do the trick. If the button inside the mouse has a slow response or doesn't travel to the 'up' position easily it may be that it has become worn. Mice are typically to cheap to have anyone fret over it to much and are the often extremely dirty without anyone ever knowing it.

Try going into your control panel, and choosing mouse. You can change the double-click speed, that may fix your issue.

I've adjusted the double click from slow, middle all the way to fast.  also note, this is the mouse buttons below the touch pad on the laptop.

Then it has most likely gone bad, the palm rest assembly's on some dells (as I have noticed) becomes stiff or flaccid over time causing problems like this. Personally, I dislike the palmrest mouse and never use it. You can typically buy new or used palm rest assembly's with the touch pad for around $30+. I use eBay and would suggest starting there unless Dell would give you a good deal. This is also assuming that the unit isn't under warranty anymore.