Typically in listings square footage, bedrooms, and features are in practice largely at the discretion of the listing agent, and the Buyers to decide what they think about it. If an argument can be made to a reasonable person that it's a 3 bedroom than it's a 3 bedroom in the context of a listing.

The only things that could override this would be like septic size constraints or perhaps claiming a bedroom in a basement with no separate egress, but typically even here as long as it's not a separate unit the interpretation of what constitutes a bedroom are very flexible

If a reasonable observer would think it's a 3 bedroom and there's no legal reason it can't accommodate 3 bedrooms then it can be listed as a 3 bedroom. The questions of valuation, and weather your property should be priced as a full 3 bedroom may be more nuanced.

Honestly though the bigger issue here, this seems like a red flag from your listing agent. I'd probably look for another realtor who's going to standup for my interests.