As windpath stated, it's always recommended to run your own local node; however, it's not always possible to do so.  There are plenty of publicly available nodes to which you can point your miners, windpath's certainly being one of the best in my opinion.  It doesn't matter which node you end up using, but you should try to find one that has been up and running for a while, has a good ping time from your location and has a low number of dead/orphan shares.

No, you don't need to change the diff using an S3+.  The node on which you are mining will dynamically assign your miner a difficulty based upon the node's hash rate.  If you look a few posts up, you'll see our Chinese friends have posted some screenshots of their settings.  By the way, congrats to them on finding that block!