You need three temples per cathedral not three for any number of cathedrals. If you are building one cathedral and want to build another, you will be able to do so if you have three more temples. However, if you are building one and want to build another and have insufficient temples to build the second one, the game only greys out the cathedral and does not tell you why until the first one is finished, after which it will give you the red insufficient message. I have always considered this a bug but a minor one, since you can just keep track of how many temples you have to know the truth.