+3v and +3v will not short.  The issue is not the power, it's drawing that much power thru connectors, traces, pad, etc that were designed for less current.  It's important to keep paths in the circuit insulated from one another.  These paths were designed and rated, intended for the load.  This is why there is a max specified load, the components, pathways, connectors, traces, pads, etc all have an intend range.

A short circuit occurs when positive finds a straight path to ground.  If there were no other paths or traces nearby, a larger than intend load could only act as a fuse, zip and the connection is gone. 
In reality, there are plenty of pathways on a circuit board. They are really really close to each other.  So, what happens... the extra load causes excessive heat, lowering resistance until an actual short occurs between positive and negative. Positive straight path to ground.

The picture, you see the middle pin above the two +3v pins?  It is the -12v.  Usually, the ground is far away from the positive in a lot of plugs.  Had the -12v been by the +12 in this case, there would have been a much larger arc blast.  See, the -12v on this plug was the heat that lowered resistance, melting and the the +3v lanes had a path straight to ground. 

+3v to +3v would do nothing.  +3v straight to negative....you see the result

Well there are a lot of reports of PCI slots crapping out, I came home to this in my rig that had 3 RX-480s in it.  They all went back for a refund today.