Application with 3 A levels only?

Have you checked out The Student room website? You'll get some good advice there.

 

The entry requirements on the website says: A*AA + STEP

It also says that A-level maths and AS Further Mathematics is essential.

As long as you have those covered then I don't see why you would be disadvantaged.

 

Yeah, but apply anyway. lots of people apply with 4 or 5 or even more A levels. You should still apply, especially since there is nothing you can do about it.

Doing more than 3 A-levels usually a consequence of going to an elite school, which gives you an advantage when applying to cambridge (they do consider the school you went to when you apply. In a fair worl they wouldn't, but they do).

 

As far as I am aware a larger proportion of Maths applicants get offers since the STEP is tricky and most don't get passed it. Basically if you have good predicted grades, a good personal statement and do well at interview then you certainly have a chance of an offer, of course you have to then get the A-level and STEP results.
You can certainly succeed at STEP if you do some practice, you get used to the questions. There are plenty of good resources for STEP lots of past papers, a couples of guides and worked solutions here: (http://www.admissionstests.cambridgeassessment.org.uk/adt/step/Test+Preparation [Broken])
if you don't already have them.
Plus I reckon preparation for STEP done before the interview would prepare you well for some of the interview questions.

 

As long as one is Maths, one is Further Maths and the other is something sciency (Physics would be a good choice) 3 A levels would be fine. I think the Cambridge maths course is very Physicsy so I would recommend Physics. I believe on their website somewhere they state that doing extra A levels beyond the standard 3 is not advantageous except for Natural Science where doing a broader range of science A levels is helpful to give you a good selection of modules in the earlier years.