Isolating one line from a multi-part score

I want to select one line from, say, a three-part score, either to make a change (e.g. add an ottava line) or to isolate it in order to copy and save it as a stand-alone single-line part. At present, I can select one line from the first staff of a three-line score that has, say, 10 staves in all, but I can't see how to apply that selection to ALL the lower lines from ALL staves in one go. Example of 3-line score attached, which I want to render as three separate parts, each of one line.

I'm sure this is a simple issue, so I'd be grateful if someone could refer me to the page in the manual that gives the relevant instruction. Advice welcome!