Trislander mixture controls
The three green knobs at the top of the panel are indeed the mixture controls - simple push/pull action. They also serve the 'idle cut-off' function on engine shut down.
Btw, there were no 'fire switches' as standard on the Trislander, though I believe engine bay fire bottles were a customer option.
Someone referred to auto mixture controls on the Dove, I can verify this as I worked for some years on the DH114 Heron which had the unsupercharged Gipsy Queens. Britain was light years ahead of the Yanks when it came to engine controls. Since WW2 most British piston engines had either semi or fully automated and inter-linked throttle, prop and mixture controls.
The Heron for example was true 'single lever' control - the 'throttle' controlled every aspect of engine speed and prop pitch setting through a complex linkage and cam arrangement , mixture being automatically controlled by a barometric capsule in each carburettor. Engine shut down was achieved by pulling on four awkwardly positioned controls behind the co-pilot seat !