If a mode lists its mode as P3, does it signify that it can not accept messages from a station using P1 or P2?
There is no specific information related to this mode although there are websites that mention it. Have a look at the websites listed through Google Search. It's the only way to find info about the P3 and P1/P2 modes.
Also, I recommend using the latest version of the software from the official website: http://www.winlink.org/
There is the possibility to contact the developer using the following email address: [email protected]
[email protected]