All three are correct, but I think most people would prefer to use either the first sentence 'Last order should be given no later than 3.00pm' or the last sentence 'Last order should be given before 3.00pm'.

The words that vary in the 3 are all correct. But 'last orders' should be plural. I assume this is in the context of an English pub? In pubs you call last orders, not give them. A barman would call "last orders" at 2:50 to tell people the bar was due to close and then call "time" at 3pm to say that no more drinks would be served.