Powering 3 DC devices from one outlet
I have never tried it on my own, but from the looks of it if one Raspberry PI drain 2A then 3 should drain 6A at max, using rough estimation the maximum power needed would be:
P = V * I ,  which is P = 5V * 6A = 30W.
Yet the source you use is
P = 9V * 1A = 9W.
Roughly it's not safe enough in my opinion, unless you could at least get yourself a 12V - 3A power supply, which in my guess won't be that rare

well, I hope it helps,
Cheers