There are a lot of things that play into power consumption, if you would like to read up on all of them you can look here: http://docs.bluz.io/tutorials/low_power/
A fully optimized bluz DK device will draw somewhere around 30-60uA with the radio off. With the radio on, one DK can draw up to 18mA when transmitting, albeit for a very short amount of time. So your peak consumption will be much higher than your average.
This is even more true for the Electron, which I believe can draw up to 2A when transmitting (you should check their data sheet to verify). So that will be your driving factor. The three DK’s should only peak at 18mA each, so call it 60mA with a little buffer.