Three is probably a good number, if they are decent engineers. Throwing more people at a problem has diminishing results very, very quickly in software, unless the project can be broken down into mostly independent teams. Even then integrating the work into something coherent can easily be more challenging and time consuming than just having a tiny skilled team own the project soup to nuts.
The number of core developers will scale with how well they can separate concerns / components. Three is probably a good number of core developers for this project, but they could probably use a fourth to do things like write tests and do things like test interoperability/compatibility.
