Not the 3 speed kick-shift, the S3X which is 3 speed, fixed gear with a cable and lever shift. Not problematic at all if adjusted properly (I've got one).
Now, your tensioner. Will that give you the range you need to keep tension when you change between chainrings?
Just thinking aloud here: With your shifts at the front, if you use a shifter that's a friction shifter, you won't need to have that extra lever below the brake lever. I'm afraid I don't know of an integrated lever (brake and shifter) that does this, maybe modify one? Alternatively, a bar end shifter would give you clean brake levers and need not be too obtrusive. Maybe a single downtube shifter if you like the look of them (you can use a band on mount if you don't have brazed on mounts).