I would have thought it would be rare - but not unknown - in the UK. Certainly I've never worked on a site without a neutral.
But if you did have a 3 wire system, then I'd suggest that the use of 5 pin sockets would be a complete no-no. If you need a neutral and don't have it, it's not hard to make your own with a 3 phase transformer.
However, on
a ship a relative once crewed on they had a three wire 240V system. Ie, 240V phase-phase, three phase, no neutral. So all AC circuits had two "hot" sides. It did cause occasional problems when someone turned up with some "4 wire" equipment as the American head electrician called it.
They also had 240V DC, and some odd colour cores. If you think the debate about Brown/Black/Grey and single pahse is "interesting" - what do you use for what when the cores are white/red/green and you are on DC ?
Actually it was 240V floating (so nominally +120/-120 relative to the hull, and int the battery room were a pair of incandescent lamps which normally glow dimply - but if one side gets earthed, then you get one bright, the other out. They had this for a while (indicating a wiring fault) but couldn't find the fault. Eventually when someone investigated a DC powered flouro not working, they found two people had different ideas over core colours when they connected the two ends of a new bit of cable
Actually, the ship had been built as 240V DC, and the DC had been retained for just emergency lighting. All the hotel loads had been converted to AC - fortunately all the breakers (not fuses !) were DC/AC so they just had to put a distribution board across 2 phases instead of the DC supply. The ship even had DC operated flouros - before the days of electronic starters - with rotary switches that reversed the polarity each time they were used to even up the tube end wear.
PS - and I was fortunate to have seen some of the old switchboards. The wiring on the back of them was a work of art in itself. Really something to behold, with neatly arranged stacks of single cores, with each clip made to fit just the number of cores in the stack at that point.