I'm a bit confused by boys3, it's a great post and has useful information but as 3 x A = 420 points the fact the average points at the top 4 universities is higher than this suggests that the people taking 'extras' are the ones that are being given offers, even if the offers themselves are directly related to 3 A level grades.
Fair point oldboots. It can be relatively easy to rack up the UCAS points as others have indicated. DS took just the three " proper " A levels, but added to that 4th AS, EPQ, and Gen Studies (which his school insisted all do, not wholly sure why) That little lot gave him easily over 600 points. He got offers from five competitive uni's, so only doing three A levels that really count, for him at least, was not a problem in that respect. He did have very high UMS scores in all 4 AS subjects - but that does not go on the UCAS application form, although some Unis, eg Cambridge, will ask for it as part of supplementary information.
I guess the other thing to point out is that most Uni's, will be making 3 to 4 times more offers than they have indicative places - Durham, for example, publish details on application numbers etc at subject level.
I'm guessing the OP's DC is Yr 12 (?), in which case I'd suggest s/he do some online research, and aim to get along to some open days in Spring / early Summer.