What is the surface that you lunge him like?? If it's a hard, uneven surface I'd be carefull about lunging him 3 times a week, but if it's a springy, reasonably soft surface you're pretty ok to be lunging that much. Also, how big is the circle you lunge him on? Small circle put more strain on his joints and spine, where as a large circle does not put so much pressure on.
If you have to space, I'd be lunging a few circles, making the circle smaller and bigger to keep him thinking, then start walking down the arena to lunging him in a straight line for a few strides, then circle again, then back down the arena etc. This is so he's not going in a curve the whole time, it lessens the strain on the joints and spine.
Also, change the direction every few minutes. I tend to go by once the horse is relaxed and responsive on one rein, I'll change and do the same on the other. Lots of changes of the size of the circle and pace will really help to build his muscle and balance.