I would say it's always worth applying if you're getting top grades. If you have mitigating circumstances make sure your tutor mentions them on your application.
In terms of applications it doesn't matter too much - I did an EPQ and found it really useful because it showed an interest in the subject and gave me something to talk about in my personal statement and at interview. I would have a look on your course page because some of them have 'required' subjects and then 'recommended' or 'suggested' subjects which are subjects you don't have to have but which they think are relevant or helpful to studying the course - that might help you pick if you decide to pick up a fourth.