I want to buy the following (emphasis mine) for my 3a+
The official Power over Ethernet (PoE) add-on board for the Raspberry Pi. Use this HAT to power a Raspberry Pi via an Ethernet cable, removing the need for a separate power supply, an ideal solution for embedded and IoT projects. For Raspberry Pi 3 Model B+ and onwards, an 802.3af compliant POE injector, switch or router device is required (not included).
I am not sure A follows B in this case. Does this device work for a Pi 3A+?