Thanks for the advice.
Now have several pointers towards a successful build, which should start next year. Will write a blog to describe progress eventually.

From memory the centre prop was used in harbour or for station keeping at sea. when required the "wing" engines were started.
I remember many years ago watching one leave Yarmouth harbour on the isle of Wight and the sound was incredible.

An interesting subject. There are details at
http://www.bmpt.org.uk/other_boats_history/Brave%20class/page_two.htm which may help your thinking.
I do believe the third motor was mainly for slow speed use but am not sure.
From a model point of view there will not be much space in a 37" hull for three motors and the necessary batteries and my 34" Perkassa runs at full scale speed on two speed 600 motors with a 9.6v 2800mA NiMh.
I have seen three prop versions but the centre prop is usually free wheeling is not driven.
If you really want performance then brushless and LiPo will give the best results and give plenty of room for 3 motors (28xx size), Escs and batteries.
I guess your next step is to source a suitable plan for your chosen variant.
I do hope you start a build blog