Upload & Sell: On
You will not need to keep Lr3 going as well as Lr4 because Lr4 can work just like Lr3 until you upgrade from process 2010 to process 2012 for whichever images you choose to upgrade. You can do that one at a time or folder at a time or all at once, but I suggest you only use process 2012 for new images until you are familiar with it. Then convert the old files.
If you do want to keep Lr3 going then it will need its own catalog separate from the copy that you will convert to Lr4. You will probably also want separate image files too because there may be a conflict with the .xmp sidecar files (if you use them, which is the default) or imbedded xmp data. When you copy the old photo files make sure that you copy the entire photo folder structure so that you need only point Lr at the top of the structure to have it find all of the photos.
I choose not to use .xmp files because they are a hassle but I keep lots of backups of my catalog.
Once you convert an Lr3 catalog to Lr4 and point it at the photo files you're good to go in Lr4 but you do not have to convert the files to the new 2012 edit process straight away. Just use it for the new photos that you take from now on, until you get used to it, and then gradually convert the old files too. In the meantime whatever you do on the old files in this Lr4 catalog will look just like Lr3 did and will have the same old adjustment sliders and so on. For this reason there is probably no real need to keep Lr3 alive other than as an emergency backup during the changeover stage.
Note that if you do convert files to the new 2012 process and if you do use .xmp files then those xmp files will be foreign to Lr3. It just gets too messy to work that way, and chances are that you don't really want to run two Lr catalogs with duplicated images and two separate versions of Lr at the same time.