How to connect 3 slave devices to one master if there are only 2 chip select pins
Two bits is enough to encode the 4 states you need (each of 3 slave chips selected, and none selected). You can use a 2:4 decoder with active-low outputs such as the SN74LVC1G139. You may need to modify the software SPI driver to use an encoded CS output instead of the standard "one hot" style.