Hi there!
I do Italian Ext, but assuming that Japanese ext is the same sort of thing, this
should be relevant to you. Do you have to do a 3 minute monologue on an issue?
Anyway, I think that the best thing you can do is be really really well practiced. You basically need to have already done a monologue on pretty much every topic they're likely to have asked you about, so that a) you will be familiar with the relevant vocab and b) you will have already sat down, considered your opinion on this issue and nutted out some main points and an example or two. For example, there are topics such as technology, racism, bullying, multiculturalism, freedom, disability, family... that are fairly likely to come up, and this way, whatever Q does come up, you'll be able to think "oh, ok, this is the 'technology' question, or this is the 'importance of family' question. Easy." rather than going into panic mode and consequently stuttering and not being able to think clearly.
Our teacher has given us loads and loads of sample monologue questions, and for our half-yearly she even gave us a list of 7 Qs and told us that she would choose from that list, meaning that we prepared those really well. Even if you aren't that lucky, if you prepare lots of topics in advance, then at least one of the 2 options given in the exam should be similar to one you have prepared for.
That said, your very first sentence should be "I agree/ disagree that... [rephrase the question here]", so that you are definitely addressing the specific question. Following that, it's a good idea to have a
brief outline of your main points, and then launch straight into it!
3 minutes really
isn't a long time. I always find that it goes really quickly and it's hard to stay within the limit.
I have to go now, the bell for the end of school is about to ring (yay! first day back wasn't bad at all!), but I might write more later. Hope I've helped a bit!