Usage of multiple DACs
The reason most DACs and ADCs have a 3-wire SPI interface is because they are unidirectional devices; some ADCs may have a few registers that can be written to in order to control e.g. input gain, but at their heart ADCs are read-only and DACs are write-only. (Note that the 4-wire mode described in the ADS8319 datasheet is not 4-wire SPI, it is a 4-wire control scheme that only uses SPI on 3 of the pins, with the fourth acting as a form of latching signal.)

Multiple SPI devices on a single bus usually means multiple nSS signals. This is shown in figure 54 of the ADS8319 datasheet as already noted. These signals can either be tied directly to an output from the MCU/DSP/FPGA/etc. or they can be derived from a demux such as the 74HC138 in order to save pins. You may be able to daisy-chain devices as shown in figures 57 and 59 of same, but it should not be assumed that any given device supports this.