I'm not familiar with the A3 Pro, I have an older Apex Jr. That said, I guess it depends on what you want the Apex to control.
The WXM is to control Radions and/or VorTechs. The ALD is for leak detection. The PM2 is to monitor salinity.
My understanding is the A3 Pro has the ability to monitor salinity but, you have to purchase the probe separately. I don't think the A3 Pro has the ability to control Radions/VorTechs or do leak detection without the WXM or ALD but, I could be wrong.