Well, if it is only for three weeks I would have them provide the hay and grain. If they are on different stuff then yours, it would not be worth the time to move them to yours, because by the time they are on your feed 100% then it will be time for them to leave.
I would for sure charge for the time it is going to take to take care of them. Are they going to run with your goats? Do you know the person that owns them? I would worry about CAE. So you will have to take all that in account. I would say if it was me I would charge about $75.00. $25.00 a week, that is IF they provide all the hay and grain.
Well I don't know if this will help but I'll just throw my 2 cents in.
I bought 2 of my goats from a friend and I used to have her sit them while I was on vacation. She charged $2.50/goat/day and she provided all of the food and they stayed with her goats. That was more of an informal thing so I don't know. I guess it depends on how much work it will be on you - if she behaves herself and gets along with your goats and stays with them, or if you have to a lot of extra work. When you think about it, that is a really nice deal for both of you, because when we board our dog, it's about $20/day.
The previous owner of the doe I purchased wanted to charge me $2 a day this year. She fed the doe (local grass hay and maybe supplemented her with grain?). She kept the doe for over a month because she was to busy and didn't get her bred when she came into heat the first time. Thank goodness she didn't charge me the $2 a day. But the doe did loose a lot of weight during that time. In the past when I would board does for breeding and ended of keeping the doe for at least a month, I made sure that the owner of the doe brought the grain and hay that the the doe was used to. Back then would charge $1 a day. The other thing to consider (i'm a worry wart) is what happens if the doe becomes injured or ill in your care? Animals stress and people are "funny". Protect your self with emergency information and what the owner wants done if something should happen accidently. This could be a whole other topic.