No, it means that on any random week, day or hour three of every four people you meet will be on a diet.

Normally I would say this as: At any given time, three out of four people are on a diet. (This means "at any random time and date you choose".)

No, it means "at any point in time", or "anytime you ask the question of how many people are on a diet". By way of example, imagine there are four people, called A, B, C and D. On Monday, A,B and C are on a diet. On Tuesday, A,B and D are on a diet. On Wednesday, B,C and D are on a diet. On Thursday, A, C and D are on a diet. So each day, different people are on a diet. It is not the same 3 people on a diet all the time. But at whichever point in time you ask the question: "how many people are on a diet?", you always get the answer: 3 out of 4.