I’ve a raspberri pi 3B+ with HASio installed on a usb-harddisk (usb 0) and at the moment p1 electricity meter connection on usb 1. The raspberri has 4 usb-ports at the moment 2 unused. You can see I am a beginner 
Now I want to connect RFXcom at one of the free usb ports. But the 2 now available ports are not recognised. Does not show in the log. Even when I put the P1 usb in one of the free ports that does not work either. How can I activate this 2 usb ports?? So I can connect rfxcom tranceiver to connect to HAS?
The Pi3b with only 1gb of memory will give you unreliable results. It is no longer on the recommended list for running Home Assistant.
All 4 ports should be available, however you are likely running into a power problem the little Pi3 power supply may not be big enough to handle another device.