OK, Are you leaving the eggs for the full 3 weeks? 21 days and I usually leave them up to 25 days.
When I let a broody hen hatch out eggs I first make sure she is broody and committed to hatching.
Then I decide how many eggs I want her to sit on. Anywhere from 8 to 10 assuming some will not hatch.
I mark the eggs with a pencil on the narrow end of the egg in case other hens decide to lay eggs in the broody nest.
I only touch the eggs if other eggs have shown up in the nest and I need to take out un marked eggs. About half way through you may need to re mark the eggs in case the pencil marking got rubbed off.
If your hen is sitting on them almost constantly, and if you are following the above instructions then you should get some hatching chicks.
If you are doing all of those things and not getting chicks then the eggs are not fertile. Either your rooster is old or not getting on the hens.
I hope this helps.