It sounds a lovely idea but you need to be able to justify it with learning objectives based on the year 3 curriculum.
Do you have to do any particular subject? I had to do mine on 3D shapes but my generic advice is:
Have a clear learning outcome and share this with the children ( I had a laminated WALT board).
Take a full lesson plan and provide one for the person who is observing you.
Ensure you have multi-sensory activities.
Have an assessment sheet that you can quickly add names to and be prepared to refer back to this in the actual interview.
Take any resources you will need and take some stickers or be prepared to give our team/ house points ( that my school, in the most recent round of TA interviews, a candidate ( who was an actual teacher) arrived nothing but a scrunched up worksheet which he expected me to photocopy for him).
Ensure the activity is differentiated but be prepared to quickly assess the level of the children and go with the most suitable task (eg I had 3 levels but only ended up using the top 2).
Ensure you have planned for longer than your allotted time to make sure you don't run out if the task takes less time than you thought it would.
In the actual lesson:
Provide stickers for the children to write their names in so you can address them by name.
Do a short introduction and short plenary at the end.
Give as much praise and reward as you can and thank them for their time and say how much you've enjoyed working with them at the end.
Make sure you do a quick assessment and record it in your sheet.
Hope this helps a bit, sorry if it sounds like a list of demands. It's not, it's just advice and you can feel free to ignore anything you don't think is right!