Wouldn't a good start be to narrow down the type of engineering and look at the league tables and email the top ten asking for typical A level subjects?
You don't even need to email. Just look at the "entry standards" on the Complete University Guide and divide the totals by the points awarded for an A. It is pretty obvious that many successful applicants will be offering more than 3xA, though obviously you can't tell whether it is EPQ, extra A levels, dance music or whatever.
You don't NEED more than three, and three will be readily accepted, especially if he is coming from a school that does not allow/encourage four. However it is worth considering:
- that places on top engineering courses are VERY competitive. You will never know whether showing that you have the capacity to take four will land the offer. Especially with double maths, where strong mathematicians find the workload for the two A levels are nothing like it is for say, art and French.
- The more doors a DC keeps open at this stage the better. Their interests evolve and change. It is not just medicine that will not accept double maths. Economics at LSE will not either (and don't demand economics A level.)
- Really really important. Lots decide to start a fourth and then find maths difficult so drop the FM. If he does is not expecting a 9 at GCSE he needs a plan B just in case. The A/A* in FM can be really hard to achieve if you are reaching your maths ceiling.
- Keeping busy, and getting used to organising a workload, is good training for University, especially if you are heading for one of the very top ranked courses where other students will be working very hard. That busy may be having a Saturday job, or doing lots of music or sport, but it's the ability to juggle that is valuable when you get to University. Don't forget that getting the place is only the first step.
I always find blanket statements like some of them do 5 and I think 4 is encouraged. Massively stressful, no outside interests and not many oxbridge offers either! borderline insulting, as if they are Johnny No Mates who take the extra A level because they hove nothing else to do. Both mine did five and did not seem particularly stressed. DD was also a sports captain and prefect, did weekly volunteering, trained with a County sports team and played for a good club, whilst DS found plenty of time for computer gaming and played for a school sports team. Both were in nice friendship groups. Looking back I realise that both were well organised and simply got on with homework, content with "good enough" rather than trying to be top of the class, and never seemed to have a problem with concepts. Their peers were doing the same so they did not feel left out/different/swotty. DS, when he was quite young, told me he had cracked how to do well at school. Simple really: "pay attention in class". If you can do that and can pick up concepts, particularly in maths and chemistry, quickly, and are reasonably well taught, four should not be an issue, and good training for University.. And if it is, cutting back from four to three is much easier than switching from fm to chemistry. (Oh, and plenty from DCs school went to Oxbridge.)
That said, it appears that OP has already made up her mind on three and is simply looking for confirmation. I would argue that with overseas students, competition for places on top engineering courses is much stronger now than when the various DH's were looking. I completely agree though that you don't need to go to a tip top University to be an engineer. Surrey for automotive engineering etc.