I want to move my presonus one 3 from Macbook Pro ( yosemite 10.5_) to my macbook ( Yosemite 10.10 )- how do I do that?

You do not need to move anything. Simply reinstall the software by logging in to your account on the other computer (at my.presonus.com), click on "My products", then "Software".

From there you'll see Studio One. Click on it and you'll see the download buttons.