Firstly, well done on getting the call for interview!
I've been teaching yr 3 for the past year, and have myself just got a new job, so hopefully am fairly fresh on the whole interview technique thing (!).
Safeguarding is a MUST - very straightforward, but make sure you know what you would do in case of a CP concern.
They will most likely ask you about behaviour management strategies, as well as your strengths and weaknesses as a teacher (don't say none! - we all have them!).
If they decide to get y3-specific in their questionning, they may ask what you would do to reverse the trending slump in attainment from y2.
In terms of questions to ask - hard to say without knowing the school. It's good to go with one or two up your sleeve, even if it turns out you don't need to ask them. Ask things that are of genuine interest to you. Have a look at the school's website - familiarise yourself with ethos/plolicies/practices/ofsted report.
Best advice, though, is to be yourself. They know what they want, and don't get too hung up on whether you are or aren't it! The school needs to fit the teacher, as well as the teacher fitting the school!
Good luck!