That is valid as an opinion, but do you have supporting evidence (or even circulated rumors) that the third prototype will appear in 2018? I don't think SAC would pre-emptively develop and build a naval variant using its own pocket money without at least some form of PLAN backing to it.
There have been lots of rumor and speculation, but the best evidence extent are
1. the very existence of the initial J-31 proof of concept aircraft.
2. It's appearance, and performance at Zuhai 2015 as a "flying display".
3. The Mock-Up of FC-31 and its active Marketing at Zuhai 2015 by AVIC.
4. The subsequent first flight and continued flight testing of FC-31.
5. Continued marketing and development of the FC-31, which requires both a substantial amount of money and effort by Shenyang, who are responsible for producing and testing China's first active duty carrier aircraft..
6. The F-31s basic configuration and suitability to operate off the Ramp of the CV-016 and CV-017, as a follow up to the J-15...
7. A perception that China desires to have an aircraft to counter the Fifth Generation F-22, F-35 both of which are currently operating in the East China Sea/Sea of Japan, and actively flying out of Japan and South Korea..
8. work continues on the JC-31/FC-31, and a third flying prototype is forecast to be following the earlier efforts in the near future.....
