and excuse me for this question,the RG402 means Grumman G-402 or not ?,and thanks.
No, that refers to The NARA (US National Archives and Records Administration) filing system - in this case, Records Group 402.
I believe that our missing "fourth" are the Fairchild M-291/M-394 designs based on the Fokker F-27 which was being license produced by Fairchild. What is puzzling is that the Fairchild M-394/M-394A docs are dated well after the the P3V was selected. My suspicion is that the M-291 was possibly for the TS-146 competition (in 1958) and the further M-394 work was for either:
a) some subsequent requirement (TS-xxx?) for some sort of shore-based S2F-class+ replacement
or
b) unsolicited Fairchild proposals.