The most common viva situation in the UK includes two examiners, one internal and one external. Some universities have independent chairs to steer and confirm the process, and in most cases a supervisor is allowed to attend with the candidate’s approval, but there are nearly always only two examiners.
There are good reasons for exceptions. It could be that the research requires people with different research backgrounds and interests. A third examiner might be needed so that certain knowledge can be brought into the viva. Or perhaps the candidate is also a staff member at their PhD institution and a second external is required to ensure that the viva is seen as fair.
More examiners could mean more questions in the viva; more people talking could mean the viva has more hours than most.
But it doesn’t mean significantly more work in preparation. An extra person won’t take long to look into: a few more papers to consider, a little more thought to consider what they might be interested in.
A 50% increase in examiners doesn’t lead to a 50% increase in prep, questions, viva time or corrections!