"Two 3rd rates in development" means something like: Agamemnon and Wapen von Hamburg III.
Wapen von Hamburg III has many guns to fit in English 3rd rate classification or 4th rate.
It is a "convoy ship", something like a fortress with sails and it could be 4th or 3rd rate.
Christian IIV could be 2nd according to English or 1st using Dutch rating system.
and yes, there are other ships in production