Hi. For PhD interviews do they ask any questions about your interests? Or is it mainly about your 3rd year project and why you're interested in doing a PhD? Thanks

Search the forum archives, plenty of helpful info there.

Ive been for a few interviews and they vary greatly. Ensure you understand the project as he/she will probably ask about your understanding of it. They will ask about your final year project, what scientific techniques you have mastered etc.
As a rule of thumb, if asked about personal interests/hobbies, dont give a huge list, mention a couple. The supervisor will have gone through major stress to get funding for the PhD so will not want to give it to somebody who is always busy with personal interests. Try to get over your passion/dedication to the subject.