First of all, it's my understanding that habits are generally made or broken in 30 days, so since it's only week three, don't be too hard on yourself! Secondly, the other day I saw a quote that really spoke to me.
“People often say that motivation doesn't last. Well, neither does bathing - that's why we recommend it daily.”
I'd never really thought of it that way before, but it's true! Just because you get yourself all wound up and motivated today, doesn't mean that motivation will last through tomorrow. Whatever motivates you - pictures, stories, music, etc - I would recommend you spend a little time renewing your motivation every day. For me, aside from browsing around here on 3FC, I love the 'fitblr' community on tumblr. They have tons of quotes, tips, photos, recipes, basically everything I need to keep me focused.
That being said, we all have our not so hot days. You're doing great though, stick to it!