The mimimum age is 18 to go to university according to most university sites, so it's good to take a gap year, so you would reach your age requirement. Apply this year and defer the entry.
3 A-levels is the minimum they want for most universities. However, if you apply to top/good ones, they might recommend an extra AS. Since you only have 3 A-levels, check the universities that you will be interested that want 3 A-levels.
I took my A2s at 17, and am just reaching the end of my gap year. I would strongly advise you to take a gap year yourself. I worked in a bookshop, went travelling and got a first aid qualification. On Monday I'm leading an expedition for my Queen's Guide Award. This time last year I could barely hold my drink; now, although I can't hope to compete with the more hardcore of my 2nd-year friends, I can at least get to six units without passing out.
Taking a gap year won't disadvantage you - you'll enter University with your year group, only you'll have a headstart on the whole independance malarky. Universities like you to take one, too, since it means they don't have to get their staff checked by the CRB. I was offered deferred entry at all of my University choices, and all the interviewers agreed that it was a sensible route to take.
To summarise: take a gap year. Claim back that last year of your childhood, and have a blast.
Have you thought at all about possible universities or courses? Maybe try writing to a few, explaining your situation, and asking what they think. Personally, I'd recommend taking a gap year - not only are they pretty cool in their own right (whether you work, travel, or sit in the pub drinking Stella), but you'd be the same age as everyone else on your course, you'd have a much wider choice of universities (very, very few accept seventeen year olds), etc. etc.. Especially since your birthday is quite late in the year, so if you went to university straight after your A levels you'd be nearly two years younger than some people. You would also have the option of applying to university in your gap year, after you've got all your results (except any extra ASs/A2s you decide to take) - which could well mean unconditional offers.
About the age thing: I will only be 17 when I go to university (if I get in and choose not to defer entry), most of them will take me, as I will have turned 18 by Christmas, which is what most of them require. Would this be the case with you? For the first few months though I will need a signed disclaimer by my parents . . . check with the universities involved.
It sounds like taking a gap year would be the best option for you. You can get some work experience, and get an extra qualification. All things help whilst trying to get in. Plus then you'll enter with people the same age as you.