Building upon the success of the Islander Britten-Norman made a larger version of it. Because there was no suitable engine with sufficient power available, the company opted for three engines, hence the name Trislander. The third engine is placed in the vertical stabiliser. This vertical stabiliser is also lower and wider, with the horizontal stabilisers attached just above the middle. For the rest it keeps the characteristics of the twin engined BN-2: a high-wing aircraft with engines underneath the wings and a fixed landing gear. The main gear with double wheels is attached to the wings at the engine nacelles, meaning long struts. The nose gear has a single wheel. The wings have a large span and have the same chord and thickness along the span, except for the tips. The fuselage has a rectan­gular cross section and large windows. It is narrow and access to the seat rows is via large doors on the left and right side. 

The different versions of the Britten-Norman BN-2A-III Trislander can be identified by looking at the shape of nose.

The BN-2A-III (the III standing for the three engines?) was the prototype of the Trislander. It evolved in the BN-2A-III-1, the first production version, with minor changes only. Both can be recognised by the short nose.

These are both versions with a much longer nose than the original Trislander. As such, it has much more space for bagage. The BN-2A-III-2 is the version certified in the United Kingdom, while the BN-A-III-3 is the US version. They are externally the same as far as we know.