What kind of language and structures should I use to gain more marks for AO3 (for the AQA syllabus)?

There are different things you should consider. Firstly, make sure that you are confident with basic structures and grammar (genders of common nouns, word order, past participles, verb agreements). Try to include a lot of vocabulary which relates specifically to the topic you are discussing in your answers. Use structures that show that you are confident in your knowledge of tenses and conjugation, for example using a ‘si’ clause to involve past and future tenses or imperfect and conditional tenses. Learning a couple of sophisticated structures can help you to gain marks, such as a phrase that involves the subjunctive (e.g. ‘bien que je puisse…’).