Find all files (type f) in the current directory (.) and its subdirectories (maxdepth 3) that are larger than 0 bytes (size +0c) and not named with the extension .txt ( ! -name "*.txt") and execute the touch command on each of them (exec touch {} \;).
find . -maxdepth 3 -type f -size +0c ! -name "*.txt" -exec touch {} \;