I had a brain tumor removed 3 weeks ago and my scalp still has a lot of fluid under it and does not seem to be going down in a hurry is this normal?

Get it checked: Post op swelling should be gone. A CSF leak may be filling the space. There could be pressure pushing fluid out through the boney defect. These issues should be brought up with the neurosurgeon.

Likely: Fluid collections under surgical incisions (often called a 'seroma') can persist for some weeks. Your surgeon would be the person best equipped to evaluate whether your wound is healing appropriately or if the fluid might suggest an infection.