This is not as straightforward as you may think.
You can certainly buy a generic adapter harness which will plug into the European DIN harness and then provide you with the bare wire tails to patch into. You may even find one which plugs directly into the rear of your head unit without the need for patching.
But you'll find that your head unit still won't work properly as the US uses a different FM channel range to Europe (US frequencies are odd, e.g.102.7, Euro frequencies are even e.g.102.8). The channel spacing is also different.
The radio itself cannot be converted, but you may be able to buy an external converter. It won't be cheap, and the head unit display will still always show the wrong frequencies. Channel changing may have to be done directly on the adapter and not on the head unit as I don't think that the DEH-P3000 has any way to communicate a request for a channel change to any external device and this also means that radio channel presetting may be impractical/impossible.
Considering that the DEH-P3000 is an obsolete unit and nothing special to begin with you can buy a brand new Euro-spec head unit of equivalent quality for less than the cost of adapting your unit and it will work correctly without any awkward Heath Robinson
Sell your Pioneer back to the USA where it belongs or bin it as it's useless over here.