What is the difference with doing 3 exercises in a circuit form compared to doing 3 sets of the 1st exercises?

When I first started out I did 3 of the same exercise in a row and could barely finish the 3rd set. Now I do it in a circuit and tend to be able to get through all 3 sets of 1 circuit.

Example: Leg Press Wait Leg Press Wait Leg Press

Compared to: Leg Press Wait Leg Curl Wait Calf Raise

Thank You for your input. :–)

correct— When you do 3 sets of the same exercise with no break, using the same muscle on the same, angle, you WILL fatigue and not be able to finish the required reps. By creating circuits of 3 exercises, you are bale to continue working, without breaks, hitting different muscle groups while others rest. Make sense?