Only if you are lucky will this work as you desire. Practical implimentations of Class-D amplifiers and power supplies widely vary and this layout may or may not work.
The selection of a 30W RMS, 24V amplifier module capable of working over such a wide power supply range means the 20V supply is within specifications. Catch is the power output of a Class-D amplifier is related to the power supply voltage to the output circuitry. An amplifier capable of operation down to 3V is not going to be maintain 30W RMS output. Therefore the output power at 20V is going to be less than 30W RMS. For example, dropping a 24V, 30W RMS Class-D amplifier may decrease the output power to 20W RMS. Is the total power of 60W RMS acceptable?
A Class-D amplifier can draw peak currents greater than the simple calculation of 1.414 times the calculated RMS current from the rated output power and the nominal supply voltage. With three modules being driven by the one source all will generate current peaks that will be basically identical and additive. The power supply is rated at 5A but what happens when a current peak exceeds 5A? Does the power supply have a hard shutdown to protect itself and the load? Does the power supply foldback the output voltage until the current drawn is less than 5A? Is the power supply a simple bridge and filter capacitor(s) such that a slow-blow fuse on the primary winding just ignores any such transient? How do the amplifier modules react under these circumstances?
The Class-D amplifiers may inject noise onto the supply line. How does the power supply respond to this noise? How do the amplifiers respond to noise injected onto the power supply line?
The resistance losses in the power line from cabling and connectors is additive for the final amplifier. Are the cables and connectors suitable electrically and physically?
What is the arrangement for earth loop protection? The audio cable earthing for each amplifier and power supply earths are not a common point. This can be a recipe for noise and hum problems.
These are just some quick thoughts on possible problems with this layout. Just accept your 90WRMS design is likely to be 60WRMS in practice. As stated, you may be lucky.
Should note that with a 16V supply for a 24V, 30WRMS Class-D module the output power is likely to drop into the 15W range i.e. total output power of 45W RMS into 8 Ohms from your 90WRMS design. If the selected amplifier is capable of 14V operation into a 4 Ohm load then the rated output power from the amplifiers should increase. The realised audio output could depend on the efficiency of the different speakers, etc.