Having a single 3-bedroom flat with two door numbers on the sale deed is common, especially if the builder merged adjacent units. You've been paying single maintenance for five years, and your flat has one electric meter. The residents' association's demand for double maintenance is likely unfounded, as fees are typically based on unit or square footage, not door numbers. Check your property tax records and sale deed to confirm the amalgamation, and inform the association that charging double for a single household is unfair and potentially illegal under the Apartment Ownership Act.