Hi, Frank,
I don't think the GPIO will do what you need. I was somewhat excited when I read about chip_en register bit, but what you really want is some way to program the I2C of each device. I like Arnold's suggestion --> we get to sell another TI chip! LOL.
If we can't convince you to buy another TI IC, then you could do something like route the SCL to all three chips in a manner that you could turn off two of them while you are programming the third.
We do NOT recommend tying all three ICs to one single I2C bus. There may be some slight timing mismatches which could wreck havoc.
-d2