Yes, there is the shell function which is there to hollow out the parts. If it doesn't work, you can create a plane offset by 5mm in relation to a wall and reuse the 3 sketches by shifting them by 5 mm and then remove the material.
In fact, I too am facing the same problem and I am desperately looking for a solution that is really suitable. I've tried almost everything but nothing works. So, please, if you have found the effective solution to solve this problem, be kind and share. Thank you first.