I think you may have a broken Pi.
With nothing plugged in, measuring only 3v across TP1 / TP2 would indicate that something is very wrong.
By now, the poly-fuse will have "blown", trying to protect the Pi, but the damage has already happened.
You were powering it through the GPIO? then there was nothing to protect the Pi.
However, in all probability the SoC was already dying before any protection could have kicked in, and nothing you could have done could have prevented it
I've no idea if this could be claimed under the warranty