Anyone know of 3x7 brifters with internal cabling?
It's a hack, but you can do this with the current Claris ST-2400 3x8 levers using an alternate cable mounting position on many "classic" shaped Shimano rear derailleurs. (Make sure whatever you're buying says 3x8.)



Sheldon Brown goes into this briefly here. It tends to work fine in this combination (8-speed shifters on 7-speed.) 

Note that all shift-housings-under-the-tape STI levers use the Super SLR brake cable pull standard. They pull slightly more cable and so if you hook them up to calipers not intended for them, the mechanical advantage will be skewed such that it feels firmer but is less powerful. It's not generally a dealbreaker and there are a lot of bikes in the world set up like this, but it is noticeable and I've seen the results be mediocre enough that I tend to avoid it when I can.