Answer:
The transition that would be best to use at the beginning of sentence 3 is Although
Step-by-step explanation:
The best way to make the transition from sentence 2 to sentence 3 is "Many Americans would love to know the answer, since cell phone use is at an all-time high. ALthough Cell phone users abound, most are not well informed about the potential hazards associated with cell phones. "Although" is used to present contrast and we can see that we have a big quantity of American users who love their devices and would love to know the information but there are only a few who have that are actually aware of that information.