Hiya, echoing Flowerfairy
Three weeks is normal for them to still be nervous, it can take less but usually it takes some time, more weeks and may even take a few months. I had one that wouldn't trust me for the best part of a year and then all at once she decided she liked me and that was that! But that's perhaps unusual. Three weeks is nothing to be concerned about.
They need to associate you with nice things, so doing what you are doing with trying to tempt them with food is the way to carry on.
They will begin that association even if it takes them a while to come right up to your hand. Teaching them to recognise a few words or voice tones can help too. When I give them food I always include one word "treat". I say it a bit how they 'wheek' (I don't know if you've come across that term).
It doesn't have to be that way of course, some people shake the food container, some say 'nom-noms'... but the trick is teach them a simple language - association of sounds for meanings, but only for a few.
For the benefit of my easily spooked piggies I say the words "only me, only me", in a soft tone, one that sounds different to my normal tone of speech, and I say this when I am about to move near them. They don't have keen eyesight so it helps them to know that it's the 'big friendly thing' entering their field of view, and not some big predator instead. They know when I use that phrase that is really is 'only me' and so can relax.
When they become brave and start to be out in the open more, and/or don't back off when you approach I find it can help to reward them by doing..... nothing. It may not sound like a reward but it's good for them to learn that they can be out in the open and nothing will happen to them.
No-one can tell you how long it will take. Some have stronger prey instincts than others, but they'll get there, and when they do it will be rewarding
