If you have a policy about board attendance then you need a procedure to monitor attendance. The names of the board members in attendance at each meeting will be noted in the minutes and the nominating committee should be tracking the attendance of individual board members.
If a board member will be asked to leave the board after they have missed three board meetings then you had better remind them when they have missed two meetings. You might want to remind them when they have missed one meeting, just so they understand the compliance with the attendance policy is being monitored.
There is no point in having a policy about board attendance if you do not establish a procedure for monitoring this attendance and dealing with the board members who are non-compliant.