Can i use my existing sim?
Thinking of buying an SE3 today...Can I use the sim currently in my iphone 8?
Yes, if the iPhone SE 3 you plan to buy is not locked to any Carrier. It uses the same size SIM as of iPhone 8.
To be able to use any SIM across the globe you must have "No SIM Restriction" against Network Provider Lock. Please refer to the pic below.