I think this is the beginning of a new Yuan build surge just like the Type 054A is having now. Not likely to stop here with three subs.
If there's a 30 year service life for conventional submarines, it looks like there are quite a few submarines are due to retire.
The conventional submarine fleet has 60-odd boats, and there are still 13 Ming-class submarines in service along with 12 Kilo-class submarines.
If there's a 30 year service life, the next Ming is due to retire in 2024 with the last retiring in 2034.
The first Kilos would be due in 2024 with the last retirement in 2035
So that's 25 boats already due to retire by 2034/2035.
Plus some of the Song-class are due to start retirements by 2029.
So they are going to have to build at 2 boats per year on average just to replace old submarines.