Notmy....
The Trislander was a development of the Britten-Norman Islander. It was a relatively cheap way for B-N to develop a larger aircraft since they already had the BN2 Islander in production. The BN2 Islander is a 10 seater twin engine a/c designed for short runways. It's loud, cramped & not particularly pleasant to fly.
They extended the fuselage forward & aft (mostly forward) of the wings to fit another 4 rows of seats. The Islander's engines were already towards the high horsepower end of what's commonly available so how to get the extra horsepower needed, preferably keeping commonality with the original aircraft? They decided a 3rd engine in the fin was a relatively simple way to do it, requiring minimal changes to the base airframe. They were even able to keep the loudness, crampedness & general unpleasantness.