This is when three disciples meet once a week and pray for three non-Christians each, nine in total for a period of a month, before inviting the people prayed for to evangelistic events called nets, with the hope that they will follow Christ. The prayer and invitations are continued until those people eventually make a decision for Christ.
The number 3 is considered symbolic of "completeness"
this is supposed to be one of the teachings of Ps Cesar. You can read more here.