Why does a Python process not run at the same time, but move on to the next process after one process is completed sequentially?

import signcros
import signuturn
from multiprocessing import Process
from Multiprocessing import go

def main() :

    U_Process = Process(target=signuturn.sign_camstart())
    test = Process(target=go.GO())
    Cros_Process=Process(target=signcros.sign_camstart())

    U_Process.start()
    Cros_Process.start()

    test.start()


if __name__ == "__main__":
    main()