If memory serves me right, when the EH-101 was being designed there were no engines that would give good single performance for the weight of the airframe. Consider that a remaining engine would have to put out a pretty awesome amount of power if the other engine failed. Two engines remaining out of three would allow a pretty comfortable margin.
At the time engine failures were more common than they are now, and so this was a driving force.
And then there was always the option of shutting one down and cruising on two - the mission of the 101 at the time seemed to be more of a SH-60B loitering type than a Sea King dunking mission. It all seemed to make sense to everyone at the time.
The 101 was conceived of long before the RTM 322 was dreamed of - remember this was back in the late 1970's - early 80's.