You wrote
I have 42 problems of which i'm to do multiple of threes
Huh ? It is not quite clear what you are asking.
Are you just warning us that you plan lots of postings in the future?
Perhaps you want us to say what multiples are.
Rather than quote a definition that might sound confusing I'll just say that
3, 6, 9, 12, 15 and so on, are multiples of 3
"..... of which I'm to do in multiples of three" might be intended to mean that you do three, perhaps every evening, making 14 evenings of work.
"..... of which I'm to do a multiple of three" would mean that you do an unspecified number randomly chosen from the list ...3, 6, 9, 12, 15 and so on. This is not likely to be the meaning as you would not know which number to choose.
Perhaps your teacher has chosen a clumsy expression but intends that you attempt every third one. That would be a sensible way to proceed. Good luck.
Regards - Ian
