All PS3 titles are region free. Thats means you can play any game you purchased from Japan/EU on your US PS3.
You should be able to sign on to any PSN account (either Japan or EU) and purchase extra contents, and they should be compatible with your copy of Uncharted 3.
Yes, all of them. Even BD movies. Nothing is region locked. Even if you want to, you can't, since the specs do not let you have region locking. Unlike Xbox or Wii.
Even if the games says "region 1", you still can play it on all PS3 consoles from any region.
I just got my uncharted 3 copy and it says it's region FREE!
Does it mean that i could get dlc from all around the world?? Any Psn Store would work?
Region free means you can play it on any PS3 console regardless of its region code, but DLC is restricted to the region code of your account.
If your UC3 is region 3, you can't redeem codes / download DLC using your US account.
I haven't tryed it yet. But won't you be able to open an EU PSN account, sign in, and redeem your code on the EU PSN? Once you've downloaded the new content, play your US version of the game with EU version of DLC? Of course, this is assuming the EU PSN will let you use the US redeem code.