Spoke breakage can be caused by a number different things - everything from rocks/sticks to incorrect tension, corroded nipples, or faulty spokes.
It's not uncommon for a single spoke failing to lead to failures of additional spokes, due to the increased tension put on the remaining spokes when a single one breaks. Once you have broken 3 spokes on a wheel, we recommend a rebuild as the likelihood of repeated failures in greatly increased.
When replacing a spoke it is important that spokes surrounding the failed spoke are detensioned and then the entire section of the wheel brought is back up to tension together. It is also critical that a spoke tensionmeter is used to verify correct tension. We recommend 110-130kgf when measured with the tire off the rim.
Here are the spokes we recommend as replacements: What replacement spokes do you recommend?
Here are the spoke lengths for our wheels: e*thirteen Wheel spoke lengths