Don't wait until 3 weeks! If she's let you touch them before (aka didn't freak right out and abandon them) then you can start now. The earlier the better, especially since mum is feral.
When I had feral females I had a trick I used if I knew they'd be OK with me around their kits. I'd feed them wet food at a certain point each day and as mum was eating her yummy dinner I'd go and pet the kittens in the nest. Don't pick them up for a few days, and keep a close eye on mum. If she seems to be getting anxious, or to be very angry about you being near her kits you'll have to try a different tactic, but it works pretty well.
When they're about 3 weeks old they'll start to toddle around (my absolute FAVORITE kitten stage
) and they may try to eat with their mum. They'll just start tasting at that age, and will still be completely dependent on mum for milk, but if you've gotten her used to eating at a certain time you'll get to see the kittens start following her around.
Then comes the tricky bit. Mum will start leading them on mini adventures around 4-5 weeks old...if there's anyone that lives nearby who is a threat to the kittens they're at a very vulnerable stage and can be easily caught. Keep that in mind. (That is why I took Muffin from his feral mum at 4 weeks. I'd gotten a straight out threat from a neighbor.)
If you can it's best to keep them with mum until they're 6 weeks old, but if you need to bring them in sooner for whatever reason do it after 4 1/2-5 weeks. They're a lot more work at that age, and would do much better with mum, but they're safety is always my first concern.
Once the kittens start toddling around spend as much time with them outside as possible; that's when you ramp up your socializing. Pet them, pick them up, cuddle them, talk to them ect. You want them to be very comfortable with you so it's easy to catch them when it's time.