A few key points:
A bedroom in most areas is defined as: dimensions 8' x 8' x 8'
A window large enough for a fireman to enter
However, in older homes the features are probably grandfathered. It may be that at the time the property was constructed the standards were different, so that brings to "market value".
The true measure is "what was a bedroom in 1900?".
The other possibility is that this 'third bedroom" was in fact created after the initial construction. It is not uncommon in older homes to have additions or modifications.
In most areas when a house is built, a building permit is issued. When that permit is completed and signed off, it is recorded by the county recorder. The county assessor then bases their assessment of the property are permitted work. You should be able to go to the county recorder's office and get copies of the original building permit.
If changes are made, in order for them to be valid, they need to be permitted. If the property was built as a two bedroom, and then an owner needed a third bedroom, they had two options:
1. do the work without obtaining a permit - which means from the government's perspective the work never happened.
2. Do the worth with a permit, meaning that the work is approved, the permit completed, and then recorded.
Technically only permitted work has value. If a home had a third bedroom added, say 200 square feet, including a foundation, walls, and a roof, and the work was completed without a permit, and there was a fire, the insurance would not pay for the reconstruction of the third bedroom.
If the bedroom was added with a permit, then the assessor would have added 200 square feet to the property , and a third bedroom, and the property taxes would have been raised.
So you can tell a great deal from the permits and county records.