While a single witch is quite capable, and a coven is very powerful, most demands, for example when taking on demons, requires more than the power of a single witch, but to be more easily convened and mobile than a full coven. The answer historically has been for three witches to work together. There would not appear to be a great deal more power from a full coven than a group of three full witches.
In addition the group of three has the advantage that they can while within a circle that is moving around see over the shoulder of others and between them, they can see a full 360 degrees. If a spell has to be read out one can do this while the two remaining can be looking in opposite directions, so cannot be caught out. A single witch might be overpowered by a stronger male, but a group of three would present far too great a challenge for anyone to want to take on. It also allows in country or mountain areas in the case of one becoming unwell or injured for one to stay with them while another goes for help.
We would normally suggest that a group of three is the ideal size for a new group. No one gets sidelined, or overlooked and you have the chance of two other opinions on any topic, and two people who you can call for assistance should the need arise.
Our covens tend to break into groups of three, not paired witches as other paths use.
