Outlet w/3 neutrals, 3 hots, 1 ground?
Whenever you see neutrals pigtailed, leave it that way.  The circuit might be a MWBC or "multi-wire branch circuit".** 

Here, the more likely reason is they're using this box as a junction.  It's a good practice, and most receptacles don't have 3 attachment points.  (you can use the back wires, OR the screw terminals, not both at once.) 

In fact, don't use "back stabs" at all (where you push the wire in and it grabs). The mechanism is cheap and horrible (there are 4 on a 60-cent socket, hello). They frequently fail, causing internal arcing, and they burn and melt the receptacles and the wire.  

Do Not get clever and pigtail a 14 AWG wire to the receptacle.  As long as all the wire in the circuit is 12 AWG, you can use a 20A breaker.  If even a single inch of 14 AWG is part of the circuit, you must use a 15A breaker.  

Actually you can go the other way and upgrade the receptacle to 20A.  As a rule, those tend to be the higher quality receptacles in the $3-4 range. 



And those better receptacles often have a feature called "screw-and-clamp". It has back-holes, but they are directly behind the side screws, and tightening the side-screw is the force that clamps the wire.  These are fine. Here's a page on the difference.  

(by the way, 15A receptacles are rated 20A internally by UL requirement; they are only constrained to 15A per socket). 



** An MWBC is an efficient way to add a second 120V circuit with only one more wire; sharing neutral and ground.  Since the neutral is shared, pigtailing is vital so changing an outlet doesn't interrupt the other circuit's neutral.  New MWBC circuits must have a maintenance shut-off that disables all circuits; but "belt and suspenders" as they say.