Some suggestions

Have a catchy title...

You want to entice people to watch your video. Your title should be interesting but also clearly relevant to your central message.

Have a clear message...

You cannot tell every aspect of your study to your audience in just three minutes so decide what is the key information that they need to know in order to understand (and be convinced by) your clear message.

Practice!

You will need to write your script before you film your 3MT. Think about how you can best structure your talk to cover the key issues and engage your audience.

Practice the presentation a number of times until you get it right. Think about your facial expressions and body language.

Consider your audience.

Your video will be viewed by people outside of your field. You want to be true to your science while translating complicated terminology into accessible terms. Try your speech out on friends and family members and check if they understand what you're saying and are convinced by your key argument.

Consider how you present yourself.

Your outfit, body language and eye contact are all part of the message. So is the information you present on the slide (if you choose to include a slide). You can be whoever you want to be, so decide what best suits the presentation you want to give.

Present on any aspect of your study.

This competition is about sharing your knowledge in an interdisciplinary, engaging fashion. If you're just starting out on your studies, you can still participate. Your research proposal can be converted into a 3 minute argument for your own study. Your literature review can be a fascinating journey through the research that has come before.