You would need to do some testing ,got any electrical test instruments?
If I had to take a guess ,the 2 blacks together are neutrals ,and the single black the switched live.
But guessing with electrics isn't wise !!
Isn't there 3 reds together ,not 2 ??
The 2 blacks together should be the neutrals, maintain the current format with the 3 reds connected together (possibly in a connector you may have to provide yourself). The black on its own is a switched live and should be identified with a red (or brown) sleeve or tape.
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