The last number is the specific engine based first cronologically when the engine was introduced on the type, and then the engine revision.
A3nn-nnnX means IGW.
For example, an A330-303X is an A330-300 IGW with GE
CF6-80E1A3 engines. An A330-301 is an A330-300 with GE
There is also manufacturer code 6 for the GE
-PW Engine Alliance.