Simple learning offered by Mirror? (5,2)

I believe the answer is:

three rs

I'm a little stuck... Click here to teach me more about this clue!

This might be a double definition.

'simple learning' is the first definition.
The definition suggests a singular noun which matches the answer.

'mirror?' is the second definition.
Both the answer and definition are singular nouns.
Perhaps they are linked in a way I don't understand?

'offered by' is the link.

Can you help me to learn more?

I've seen this clue in The Guardian.

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