I was in a similar situation a while ago where I was debating with myself whether to take an extra AS or not. In the end I ended up not doing so and I got really good grades and am going to a RG uni. So I wouldn't say an extra AS is necessary. Although, that may depend on the universities you're applying to - look up their requirements. If you think the extra workload may interfere with your 3 A2 subjects, then I wouldn't recommend it.