OK, at the moment (and things keep changing, so who knows whet the system will be by the time you dcs start GCSEs) the system is as follows.
All children have to do GCSE science. They studyChemistry, physics and biology for a year, do a lot of exams/assessments and will get a GCSE in Science at the end of year 10. If they pass most schools will insist that they then go on to do a GCSE in Adittional Science in year 11. For this they will also study Biology, cemistry and physics. If they pass at the end of Year 11, they get a GCSE in Additional science. So they will end up with two GCSEs in science, that will covere Biology, chemistry and physics.
Some schools will let scientifically able children do GCSEs in Biology, Chemistry and Physics as separate sciences (just like we tended to do when we were in school). If they do this they will end up with 3 GCSEs in science.
there isn't UMHO, a gread deal of advantage in doing the 3 separate scineces with regard to A level, since we do most of the stuff, all over again at a higher level than separate science GCSE. Able children can get A grades at A level without sitting 3 separate sciences.
As a parent, and a science teacher I would encourage most childre to do the oiption that gets you 2 GCSEs in science. They can then use the 'spare' GCSE to study somethings else.....broadening their options, which is IMO a good idea at 14, because at that age few of them know what they are going to do later in life.
mant schools offer 3 sep sciences as a twilight session. This can be done, but is more work for the child, and unless they are very keen, most would rather be out playing footie etc
HTH