Very steep climb for a member of a trio (3,2,5)

'very steep climb' is the definition.
I don't know anything about this answer so I can't judge whether this works.

'a member of a trio' is the wordplay.
I cannot quite understand how this works, but
'a' could be 'one' and 'one' is present in the answer.
'trio' could be 'three' (synonyms) and 'three' is found within the answer.
The remaining letters 'in' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

'for' is the link.

Can you help me to learn more?