1- I come home from school at 3.00.
2- I come back home from school at 3.00.
Which one is correct?
The only thing about both statements is that they are potentially ambiguous. Do you mean that you leave school at 3pm and then head home? Or do you mean that you arrive home from school at 3pm (therefore you must have left school before 3pm)?
If you're talking about your arrival time at home, you can avoid the ambiguity by saying "I get home from school at 3pm" (BrE).
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.
We'd say 'I get home from school at 3 o'clock'.
Nobody thinks we mean 3am.
That's true. There's no need to specify "3pm". However, if your question was more to do with whether we say "I arrive home ..." or "I get home ...", then in BrE the latter is more likely.