Weight loss happens as a result of being in a calorie deficit, not from exercise, so from that point of view, there's no difference.
If you're eating the same amount, the 3 separate rides would probably mean a (fairly minimal) increase in the calorie deficit because you can work harder for 1 hour at a time than you can for 3, but it is minimal, and I wouldn't use it as the basis for a decision one way or the other.
However, if you're trying to use all the calories you burn doing exercise to create a calorie deficit, the 3 separate rides are much more effective because of the size of deficit you can create. If you burn 500 calories per hour, and you ride for one hour, it's fairly easy to undereat by that amount of calories in a day. If you burn 500 calories per hour, but ride for 3 hours, you'll find it hard to undereat by 1,500 calories in a single day, so you'll end up eating some of the extra calories you burned through cycling, and this will slow your weight loss.