This is pretty normal. For me the first two months were pretty hard and slow. It was like mentally my brain had checked out and wanted to be anywhere else but inside my head. Since then, energy and focus has come back, albeit in fits and starts, but it has by no means been a straight line.
My advice, don't take anything to challenging or new for the next few weeks and try to exercise a little every day, even that means just going for a short walk outside.
I'm currently at 4.5 months sober and generally speaking, I feel miles better now than I did on November 10, 2012. My days are soooo much more productive.
Hang in there, it gets better, just have to be patient. And huge congrats on 3 weeks, that is massive!