In Welsh Patagonia they only use one fly

.
Most of the early sea trout sea trout authors - prior to Falkus inventing night fishing

- wrote of 3 fly casts for sea trout.
Hamish Stuart 1917 'where it all started' in the Hebrides refered to the standard 4 fly casts and was considered a radical as he only used a team of three.
R.C. Bridgett mostly used a team of 3 on rivers all over Scotland; as did Jefferey Bluett on a variety of West Country streams - and 30 years on the Tavy in Cornwall. Maj Kenneth Dawson and L.R.N. Gray 'Lemon Gray' were other West Country exponents. I could rattle-off another 2 dozen authors if need be.
In fairness to Vass - Moc Morgan did state that he prefers a team of three small flies, to 4 lb nylon, when daytime fishing the top of tidal runs (Dovey illustrated in his book) for sewin.