There was a similar thread to this last week.
People pointed out 2 things which are both true at the same time; huge numbers of the older UK housing stock is 3 bedroom, 1 bathroom/loo in style. It is perfectly normal. AND newer housing stock with 3 beds usually has at least 2 loos and often 2 bathrooms.
1 and 2/3 loos are all very normal in 3 bed houses. As new builds make up a growing proportion of 3 bed houses, having more than 1 loo will increasingly become common.....but people choose houses for lots of reasons. Lots wouldn't consider a new build on an estate but prefer a 3bed older house without an extra loo. They might prioritise not being on an estate, being closer to well established schools, bearer town centres, near family or friends or any other reason.
Families with 2 or more children often like more than 1 loo and make it a priority and so choose newer housing stock to get it.
However, loads of people live in 1 loo households and do t see their 3-bed as inferior to the 3 bed with extra loos.
It depends on the size of house, location, other features and also price too. There will always be a market for cheaper 3 beds and more expensive ones.
Once you get to 4 beds I definitely think 2 loos and really 2 bathrooms are expected, but in 3 beds there is a real range and it's just 1 determining factor when choosing.
Personally, I'd rather live in the Victorian semi with 2 big reception rooms and a lovely garden near the town centre, and sacrifice the off-road parking and extra loos, but people feel differently.
I do t think older houses with just 1 loo will suddenly be hard to sell if they are in good locations and have other things going for them.