Hey, yr12 student here who was a triple science GCSE student, and I think so long as you have consistent grades of around 75% (That's where it was for our class- 70% physics, 75% bio, don't remember chem- it was a long time ago, but it didn't have to be exact). I presume you do end of topic tests? If those have been pretty consistently over 70% (Again, may differ depending on your school), then you'll probably be set 1. Even if the most recent ones were important, don't sweat it too much, because so long as your other past papers have all been over 70%, then you're probably okay. Hope this helps