In British English, the second is most common, as far as I know. Both are fine though. Also, you should contract "I have" to "I've," if you're writing/speaking casually.

Both sentences are grammatically correct, but they are commonly used in different regions. In British English, the first sentence is more commonly used: I have Maths on Monday morning. In American English, both sentences are used, but the second sentence is more common: I have got Math on Monday morning. It's worth noting that "have got" is more commonly used in British English in informal speech, while American English tends to prefer the simpler form "have" without "got." However, both forms are generally understood in both dialects.

Both are fine.

#2 is the one you will hear most often. However, it makes little sense. "Got" is a past participle for "get" (as is "gotten"), so "I have got" is present perfect tense. But that tense makes little sense in this context. To see how nonsensical it is, use "gotten" in place of "got". If "got" works, then "gotten" would work too because they are both accepted as past participles for "get". The sentence then becomes "I have gotten maths on Monday morning" Doesn't that sound weird? Well, if that's weird, so is #2 because they are the same. #1 is superior because it is correct and #2 is wrong.