I don't recall stating it was fact anywhere in my reply mate.
You didn't write "in my opinion" or "from my experience" or anything like that before you mentioned "3 isn't enough". That implies that what you're saying you're stating as fact.
Anyway, this discussion is pointless without knowing how many fish will be in there. OP mentioned it currently has one fish. Can a single large sponge filter take care of the waste from a single fish? Probably yes. Doesn't matter what the tank size is.
The question to ask is what the tank will eventually hold and then advise on how many sponge filters would be necessary.
You also need to know how much is fed and how often and about the frequency of water changes before advising on filtration setups.
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