It’s worth remembering that rabbits are prey animals and have a very different perspective on the world than, for example, a dog or a cat. It can take them a long time to build up trust with new people. Both my bunnies are very friendly, Sidhe cuddled up and toothpurred the first time we met her when we were being introduced to potential mates for Puck at the rescue, and has consistently been a very affectionate girl towards me in particular and others to a lesser extent – however when we first brought her (and Puck) home from the rescue after they’d been bonded, she was very nervous, very shy and wouldn’t leave Puck’s side for anything for the first few days. It took her a good few weeks to feel confident around us and several months to develop the bond we have now, even though she was very friendly with me from the outset (and hadn’t responded like that to other prospective adopters). These things just take time and patience.
Another thing to consider is her background before you adopted her. You specifically say “adopted”; did you adopt her from a rescue/shelter? (No shame if you didn’t, I got Puck, my first bunny from a breeder) If so, there’s no way to be sure how she was treated prior to you adopting her unless (like Sidhe, who’s mum arrived pregnant) she was born at the rescue. If she’s had a bumpy start – and sadly, being young doesn’t mean she’s definitely been spared that if she’s a rescue bun – it may take her a little longer to trust that not all humans are going to treat her the same. Just things to consider.
Honestly, at three weeks I wouldn’t worry that she’s not being particularly overtly affectionate; she’s choosing to run up to you, even if she does scamper off quite quickly. Bunnies aren’t shy about letting you know if you’re not welcome, and if she really didn’t like you, I doubt she’d come over at all.
I’d like to echo the suggestions above regarding just sitting in with her and doing your own thing. When I first got Puck I sat in his play pen with a book and “ignored” him unless he came looking for attention. Even then, I didn’t try to pet him straight away, I just held my hand out where he could sniff it and let him take things at his pace. If she’s nervous of the strange new human, then reaching out to pet her every time she comes close to you might be quite intimidating for her. Give her time; much like Chloe, Sidhe has become a bit of a rabbit-shaped shadow now that she’s had time to get used to me. She comes over for attention any time I go past the pen and gets quite offended if I won’t put my hand in to be groomed and give her ears a stroke XD