The main rank is outside the door of T3, i see no benefit in prebooking.
I agree with mantalk.
I don't know what the cost of a taxi from the rank is, but we prebooked and prepaid with skyscannerairporttransfers.
The driver met us at the door , walked us round the corner, and brought up his merc. There were Four bottles of water in the back for us.
It cost about 15 Euros to a hotel just by Tetuan Bridge, edge of old town.
We thought it was worth it.
Thank you so much for the advice everyone. Really good help.