/folder1/folder2/folder3
/folder1/folder2/file1.txt

/folder1/folder3
/folder1/file1.txt

cd /folder1/folder2/
mv * ../
cd ../

ls

rmdir folder2/

mv  /folder1/folder2/*  /folder1/
rmdir /folder1/folder2/

mv  /folder1/folder2/{*,.*}  /folder1/
rmdir /folder1/folder2/

mv: cannot move ‘folder1/folder2/..’ to ‘folder1/..’: Device or resource busy

$ find .
.
./folder2
./folder2/.hidden
./folder2/file1.txt
./folder2/folder3

$ mv $(find folder2 -mindepth 1 -maxdepth 1) . && rmdir folder2

$ find .
.
./.hidden
./file1.txt
./folder3