Is there a downside to trying multiple flight instructors? My local airport has a Part 61 flight school and gives students the option of selecting among three Certified Flight Instructors. Farhan and others have posted excellent explanations of characteristics to look for in instructors.

Other than wasted time and money, is there any downside to doing an "Introduction to Flying" lesson with each of the three instructors and seeing which one I like best? Similarly, am I likely to offend CFI's that I'm "looking around" other options?
No. Definitely try more than one CFI and ideally, more than one school. This is the smart choice and one I recommended regularly when I was an active flight school CFI. 

You're about to spend a ton of money - do the due diligence and spend more up front to make sure you don't waste time and money with someone whose teaching style or personality clashes with you as a student.