Crossword Genius

A pair royal (of kings?)

Ross

I believe the answer is:

three

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

'a pair' is the definition.
The answer and definition can be both quantities as well as being singular nouns.
Maybe you can see an association between them that I can't see?

'royal of kings?' is the wordplay.
I cannot quite understand how this works, but
'royal' could be 'r' (abbreviation) and 'r' is located in the answer.
The remaining letters 'thee' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for three that I've seen before include "Prime number" , "A crowd, you might say" , "Feet in 3 Down" , "Number of Furies or Norns" , "Cardinal number" .)

I've seen this clue in the Evening Standard.
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