It is "normal", for a prop to turn right handed unless specifically asked for, that is clockwise. However in the case of a twin screw vessel, the props turn opposite ways, to cancel out the 'walking' effect of the props, especially going astern, but also noticeable ahead. I have never skippered a triple screw vessel, so cannot say through experience, but would assume the centre prop would be right handed on a triple screw vessel. On Damen multicast with triple screw, I think all three engines would be run when the towing power is required, having three engines would not make an appreciable difference to speed when running at designed speed. Three engines would give the owner/ skipper more versatility when steaming from one job to another, if the vessel was not in a rush just the centre engine could be run, if normal steaming speed was required then the two outside engines would be utilised, and if power was required three engines would be run.
I won't go into fast vessels, as this is a tugs thread.
Hope this helps.
Cheers