Yes three devs for the core engine plus multiple devs for things like SDK, Documentation etc.

When talking about catapult you have to difference about the private Mijn chain and the public NEM chain. Catapult is already released for the private chain and being tested by companies. It is open source and everybody can have a first look at the impressive new Catapult architecture.

The public NEM chain will utilize all of the Catapult technology already published. But for a public chain there are additional modules needed like harvesting, node trust system, governance etc. These modules are in development right now. Once finished the public Catapult will arise. Most of the work is already done!

Keep in mind that once the public chain also has the Catapult update the two chains can interoperate with another. You will be able to make atomic swaps between the public and private chain. AFAIK NO other coin is so close to releasing this long anticipated feature. This is the final puzzle piece for NEM, and many companies eagerly wait for it to launch their products on top of NEM.

Overall the development phase for catapult might be about three Years. That is very reasonable considering all the features where NEM will be the first to offer them to the crypto world. We talk about the core of NEM not some webapp utilizing it. Any error here has massive impact on the complete NEM ecosystem. So better to handle this with the very carefully and don't rush anything