[Imp]

You can only take cover 3 times a turn. So move & use or if receive fire take the option of returning fire or taking cover.
As Andy says if you decided to take cover you will only know if you managed it next turn. If you were being targeted & they switch targets next turn you probably managed to break contact.
Much like when units take cover automatically your chance of success depends heavily on range suppression state & experience of units with an eyeball on you.
Short range attempting to take cover is not going to work.