The data sheet https://cdn-learn.adafruit.com/assets/assets/000/040/671/original/FXAS21002.pdf?1491475056 & https://cdn-learn.adafruit.com/assets/assets/000/043/458/original/FXOS8700CQ.pdf?1499125614 indicates that the device is, also, a SPI device. Hooking up 3 devices to one uController should not be an issue using SPI. Next each device on the module, there are two of them, have numerous registers located on each device.
For instance the FXOS8700CQ has a register called CTRL_REG4. In order to change that register you'd, look up that registers bit descriptors and set up the desired bits to change, and write the information to that registers address. Quite frankly, you'd want to use a pre-written library and use that libraries feature set to change that register, if you are unable to write I2C or SPI code yourself.