Personally, I see no real need to have a different name for YA and picture books unless your YA is incredibly edgy...in which case I see no need to have a different name for YA and the novels. So you'll "need" two names, at most, in my opinion. Depending how your YA reads (a lot of YA literature reads close to many novels), it would take one of the two names for the novels and picture books.
But again, agents/editors will likely be your best resource - they know marketing far better than I.