How do I calculate the wattage of a set of speakers?
I assume you have speaker elements (no cabinets) which are intended to be used without cross-over filters, they are not bass nor treble elements.
If we assume the 5W and 10W ratings are valid with the same signal content you can connect the 4 Ohm speakers in series and insert the 8 Ohm speaker in parallel with the series circuit. Then you have 4 Ohm load which hopefully stands 20W total input. Now you need a single amp which can output that 20W to 4 Ohm.
Speaker sellers unfortunately do not specify what the power rating exactly mean. There are some norms for it or the rating can be R.M.S but as well there can be said nothing else than a number of watts. This gives a possibility to write as high watts as needed for good looking ads as long as the photo of the product is plausible when compared to known speakers.
The watt rating principle is not the only unknown. The functionality of a speaker depends heavily on the cabinet, speakers convert electric power very differently to acoustic power and behave very differently as electric loads. Given 8 or 4 Ohm can actually be far from the truth because speakers are not resistors, they are electromechanical machines.
=&gt; Constructions based only on given watts and ohms are gambling.
The gamble can be successful. I have built nearly the same combination for a guitarist. I put the speakers in the same cabinet which had open back wall. The guitarist liked the sound and that's that - well done! I guess nobody would stand the result if the signal was recorded classical music.
BTW. Test with a few volt battery to make the speakers to push air to the same direction.