I am not a teacher.
The way 1. is written it sounds like one exam treating three subjects.
If they are distinct separate exams then I would use the plural.
Logic dictates that for there to be an average there must be more than one quantity involved. If it were just one exam your score would be 80, not an average of 80.
Therefore 2. is plural. At least it is in BrE and since you used "Maths" we can safely assume you are using BrE.
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