On the day of the wedding, how early should the ushers arrive at the church? For instance if the ceremony is at 3pm - should the ushers arrive at 2.30pm, or earlier, or later?
I have checked out several wedding etiquette books/websites but haven't found this mentioned.

The Ushers should really be among the first people to arrive, to sort out any last minute hitches and make things are as they should be and, of course to greet the first guests. For a 3pm Wedding they should be at the church at 2.30 at the very latest and ideally quarter of an hour before that.