Weak pull down for SPI
You shouldn't need to pull down the MOSI or Clk lines.

Master Out Slave In is an output from your controller, as is the Clk line.  Unless you do something that turns those pins to inputs, they should be just fine without either pull-down or pull-up resistors.

On the other hand, Master In Slave Out is an input to the controller.  When none of your ADC chips are selected, that line does float.  It needs the resistor that you added.