Hi! I did my GCSEs a few years ago. My biggest tip would be to choose subjects you actually enjoy, as you'll be more motivated to revise, and as a result you'll come out with higher grades.
I would say to go for triple science over combined. The content is mostly the same, except you learn a few extra modules with triple, but these extra modules are super helpful if you go on to study any of the 3 sciences at A-level. I did triple, and then studied A-level chemistry and biology, and found that I had an advantage over my peers who did combined. The other advantage with triple is that you'll get three separate grades, whereas combined gives you two grades, and it's calculated as an average of the three sciences. So if you're really good at biology and physics but weak at chemistry, you could come out with a 9 in biology, an 8 in physics and then say a 6 in chemistry, whereas if you do combined the poor performance in chemistry could drag you down to maybe a 7-6 overall. You also still have the option of doing foundation or higher with triple, likewise with combined.
Choosing between IT and computer science. Computer science is more respected as a GCSE, and will give you the foundation to then study it at a level, but IT would probably be an easier workload. Assess your priorities - if the other GCSEs you've chosen are the 'harder' ones, then maybe go for IT to give yourself a break, but if you've chosen other BTECs/certificates, then go for computer science. Your school probably has the option to switch for the first few weeks anyway if you decide you made the wrong decision.
I did fine art at GCSE and it was LOTS of work if you want a good grade. I got 9s in all my GCSEs apart from fine art, which I got an 8 in, but I probably put more effort/time into art than any of my other subjects. There also isn't a huge amount of freedom - your teachers tell you what kind of art you have to produce, which artists you have to study, and people also don't realise that there is a lot of analysis too - you spend quite a lot of time writing about your work, or writing about other artists' work. Also, the marking can be quite subjective. It's internally assessed (unless you get moderated) so your teacher gives you your mark. However, it was quite an enjoyable subject. All my other GCSEs were very content heavy, so it was a nice break just being able to listen to music and paint. I have friends that did media/DT and they seemed to have a more manageable workload, however, they were BTECs, so are worth slightly less than a GCSE.
I did history, which I personally really enjoyed, but you've GOT to enjoy writing and analysis, or be willing to put a lot of work in. You also have to dedicate a lot of time to revision if you want a good grade, and some of the topics can be dull and repetitive. I also did geography, which I'd recommend - a super easy 9. As long as you're able to apply knowledge, you're guaranteed a good grade. I didn't even learn most of the case studies, I just used examples that I'd read in books or seen on the news, and I came out with a 9.
I have a lot of friends who did sport and they all came out with the highest grade. Minimal workload, and also sounds quite fun if you enjoy PE.
Good luck!!