As a parent of someone else doing Triple Science, I'll tell you what I tell him (& your school may have already said, I know his has) - you would not be entered for Triple Science if your teachers did not know you can do it. They would have changed your exam to Double Science so that you would have a better chance of passing the exams only concentrating on 2 exams rather than 3. They could have changed it after your Mock Exams still, that's 1 of the things Mocks are for. This also applies to Maths - you can do it.
So, that's my first advice: Relax, and stop thinking you can't do it, because you can do it.
Second: as souffle said, lots of water, lots of sleep. Also plenty of good food - you know what's good for hamsters - the same applies to us humans. Eat good quality balanced meals. Have breakfast before you go to school for the exams, take a bottle of water with you & snack to have between or just before you go in, have lunch in between and eat properly in the evenings. It will make a huge difference to your concentration when you are properly fed & watered as opposed to your friends who grab a mars bar & can of coke at lunch time after skipping breakfast because they got up late.
Thirdly: Have you seen a doctor about the potential panic? Elder son has asthma & takes his inhalers into the exams, my friend's daughter has OCD and has a slip that she can hold up to excuse her from the exam to go the toilet (she won't actually use it because with the OCD for her comes a hyper awareness of what other people might think, and she can't cope with being escorted to the loos by a teacher & having everyone know that's where she's been). But there are ways the exams have to be adapted to people's individual needs. With the support of the GP or other professional.
Ok, revision advice now: Look at what you got from school - if you got books for your subjects, start with those because they're for the syllabus you've been taught, for the exam you're going to take. Did the teachers recommend particular websites? Ask them if you can't remember. BBC schools site is good
BBC - GCSE Bitesize - Homepage As suga said, make a plan and stick to it & reduce the distractions.
Make time in the plan to chat to friends on facebook or play minecraft or whatever works for you, and limit it to those times! I was lucky because I did my exams before mobile phones, so if someone phoned on the landline, mum could say 'She's doing revsion, she'll phone you back.' So turn that mobile off & log out of facebook! If it's an emergency, someone will get hold of you another way. If they didn't need to get hold of you another way, it wasn't an emergency, so could wait till you were ready to talk.
Everyone is stressed right now: you & everyone else doing GCSEs, A Levels etc is stressed; the parents are stressed because we know it's important for you to have all the choices of what to do next; the teachers are stressed because the good grades reflect their good teaching. But in ten years time, the exact grade you got won't be important, passing at C or above will be. Your attitude to the exams is as important as knowing the answers.
(Oh, & I guess you've already got the pens etc prepared, but you are best with a clear pencil case containing ruler, pencils, black, blue & red pens, pencil sharpener, compass...)
Good luck!
(And apologies for the essay...)