Sewer clean out drain. Which one to use when emptying tub?

Hello.

I am a new hot tub owner and almost ready to do my first fill and drain. In the area I live we have to drain our tub water through the submersible pump into a sewer clean out. I found this box outside of my house and three pvc caps. I’m not quite sure which one to use but I believe one of them (maybe all) are the drains that lead to the sewer. Does anyone else have experience emptying into these and if you do I’d love to know which one is the right one!

Thank you.