Yes, you can still do sciences at A level if you don't do triple, although you will find A levels much easier if you do, particularly chemistry. You are unlikely to be at a huge disadvantage though and should be fine if you work hard.
Generally I would advise drop to Trilogy (are you doing AQA?) and get a better grade, unless you know you want to do A level, in which case the potential lower GCSE grade is worth it for being better prepared for A level.
I would advise you to not even think about doing A level Sciences without being able to get a 7-7 easily in trilogy or a 7 in the science/s you want to study and at least a 6 in the others if doing separates. I'm a Science teacher, do feel free to ask me any questions.