Explain Why These Sentences Are Ungrammatical And Give The Correct Sentence?

Here is the full form:

3-We are more hardworking than they [are].

Thank you for answering Mister Micawber ! but i still don't get why this sentence is incorrect. If you would kindly explain just this sentence it would be wonderful.

We are more hardworking than them.

↑ It seems this sentence isn't incorrect too just because it's not in its full form? tia