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Journey allowed three similar things (7)

Ross

I believe the answer is:

triplet

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'three similar things' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

'journey allowed' is the wordplay.
'journey' becomes 'trip' (trip is a kind of journey).
'allowed' becomes 'let' (to let is to allow or permit).
'trip'+'let'='TRIPLET'

Can you help me to learn more?

(Other definitions for triplet that I've seen before include "One of several arriving together" , "Group of three equal notes" , "Set of three things" , "One of a set of three" , "Handful of notes" .)

I've seen this clue in The Sun.
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