I have one spare 140mm fan for case.

[AllYourF0rt]

So I have a 140mm fan. I want to put it in muh case not sure where. I have 1x140mm in the front for intake and 1x140mm in rear for exhaust. I have room for 1 more in the front or 3 on top for exhaust.

[ccokeman]

Up top is my vote. Heat rises making it easier to cool your components. 

[SpikeSoprano]

+1 with what coke said, mount it on the top .

[cchalogamer]

I'll be different and disagree 

 

Generally I prefer positive pressure in a case allowing you to filter incoming air and keep things cleaner easier, so adding another intake would win out for that one. As for heat rising...true though it doesn't really happen fast enough to make a real world temp difference unless you're using mostly passive cooling. The air will still find its way out of the case with the path of least resistance using another fan to blow in.

 

But don't take my word for it, install it in your case and test it That's what I did and as far as cooling similar fan adjustments didn't change temps for me EXCEPT swapping to positive pressure helped with dust build up. You may get a different result but all you need is some time to swap things around and test it for yourself.