3-in-1 Vs Shop Vacs
Interesting comment that the 3-in-1 cleaners were a progression from the shop vac / wet & dry cleaners.
In fact, if we take Vax (what one might call the original domestic 3 - in - 1), this was first on sale as early as 1978, during which time the only others were the UK versions of the US shop vacs, known of course as Aqua Vac. When Vax started to hit its stride around 1983, the range of wet & dry machines was becoming larger, with Hoover having a Shop Vac model branded for them, and as mentioned before, Rowenta, which offered a much quieter and refined cleaner, more suited for the home as the neat all-plastic construction looked less industrial I suppose.
I think it was the boom of sales of 3-in-1 cleaners which possibly caused the sales of wet & dry cleaners to rocket too, as in some respects they may have been seen as "almost" the same as a 3-in-1 by those on a budget who lusted after such things, but of course were considerably cheaper to buy.
I think also that the physical size of the cleaner compared to the low price tag may also have been tempting to those who percieved them to be lots of cleaner for the money. Frankly I am astounded as to why anyone would wish to drag a wet & dry cleaner round the house, but as was said already, they were incredibly popular for ordinary in-home dry use.