Lin could have squandered the opportunity that we have never noticed. But he made it most of it. You never know when opportunities are going to arise in life. Often, they're when you least expect them. Make the most of them. Don't fritter them away.
NOT A TEACHER
I'd say that you can't use "fritter away" with "opportunity". But even if you could, "miss" or "throw (away)" or "loose" sound more natural.
Like SD and 5jj, I disagree. And I think the verb you are looking for is 'lose' (/lu:z/) - which unfortunately for students, doesn't have the same vowel sound as 'chose' and does have the same vowel sound as 'choose'; and 'choose' doesn't have the same consonant sound as 'loose'.
The first sentence. It could mean 'Lin could have squandered the opportunity that he had never noticed.' But other meanings with different corrections would be possible,
The second sentence is OK.
In context, the 3rd sentence might be regarded as acceptable, although 'they're' means something like 'they happen/arise/occur/turn up...'.