3% isn't quite right, but most beers were between 3.4% and 3.6%. Take Heineken, for example. It was brewed to the English palate with an ABV of 3.4% from the 1960's until the early part of this century. Heineken Export, which was the higher ABV, 'continental', more authentic version of the product wasn't introduced alongside it until 1991. The same went for keg bitters, which were between 3.4% and 3.8% during the same period.
Through the 90's, the ABVs of 'standard lagers' generally rose - I think Carling went from 3.8 to 4.1, for example, and after a number of years of decline, in the middle of the 'naughties' mainstream lagers like Carling and Carlsberg started to increase in volume for reasons that I've never quite understood. That was short lived, however, as those core brands are haemorrhaging volumes to 'craft beers' and are now suffering from existential crises.
The reason the British palate was so attuned to lower ABVs was (I think) because of the preponderance of heavy industry in the middle of the last century, where drinking large volumes of beer at lunchtime was commonplace. I've also got a feeling that ABVs were routinely and consciously reduced in WW1 in order to keep munitions production on track and they never went back when peace broke out, not least because the duty was lower and it suited the brewers to maintain the status quo for financial purposes.