Yes, it's OK. You can put the punctuation marks after 'a' and 'm' if you like, but not everyone does. Either 3 a.m. or 3 am is acceptable. Don't put the colon : between the numbers, though.

I usually write "3 am" - this is normal. If you want to use the 24 hour clock e.g. for professional reasons, you could write "03:00" (you don't need to add "am" here)

Either way is correct, 3 am or 3:00 a.m.