How near is the residential location?
My son's school usually do a residential trip of similar duration, which has lots of outdoor activities on during the day. But this is in Year 4 rather than Year 3; there were still quite a few children who were nervous about staying away from home. At that age quite a few children have difficulties with being dry at night too.
Our school's strategies for maximising the number of children who were able to go were: visiting the location area beforehand - a few children did this and felt more comfortable knowing in advance where everything was, how things were done like the meal set-up etc. Others were fearful about staying away from their parents and in those cases, they had permission to be collected last thing and taken home then brought back in the morning. This really helped that they didn't miss out entirely with the daytime activities but had a really positive, non-homesick experience.
The children could telephone their parents if they wanted to.
The centre staff are used to helping anxious children and have a lot of experience in assisting those who need help with staying dry at night.
Can you speak to the school and ask staff how they will helping the children with their anxieties? There is a lot they can do to make this into the positive experience that dc will remember fondly for years and your child will not be alone in being worried about it. (Doing it in a year or two years instead would be a start...)
Don't keep your children out of school for this. If you can persuade your dd that she will have a great time, she will learn so much, and she will also learn, maybe, that sometimes the fear of something is far worse than the thing that she is fearing. Agree with pp, these trips can be so good for building confidence and if it's linked in with a lot of the work they're covering, she will miss out in many more ways.