Three banks combined to further their progress (8)

I believe the answer is:

triremes

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'three banks' is the definition.
The definition and answer can be both man-made objects as well as being plural nouns.
I also know that 'three banks' relates to this answer.

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for triremes that I've seen before include "Galleys" , "boats" , "They had three banks" .)

I've seen this clue in The Globe and Mail.

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