Chat GPT tells me that a third DI is overkill and not worth it due to the laws of diminishing returns.
Also for the original poster of this thread, i think it would be impossible to answer him as no one knows what his TDS was when his membrane was new. Maybe he only ever got a TDS of 20, now its 22 so its fine for ages yet. Its impossible to say without more information when he needs to reenew his membrane.