Logic level converter for SPI
Pick the highest-voltage buss as your common point.  Then simply use a logic-level converter between that buss and each individual voltage level.  

There may be additional effort required if you have multiple devices talking to the RPi.  Specifically, you need to enable only driver talking to the RPi's receive pin.  This is not difficult but you need to be aware of the problem.  In general, use the enable line for each SPI device to also control the TX output enable of the level converter.  TX, in this case, refers to the pin that is feeding the RX pin on the RPi.