What does 3PSP mean in box 14 of a W2 ?

You have to ask your employer what it means. There are no standard rules for what goes in box 14, and no standard codes or abbreviations. The employer can put pretty much anything they want in that box. If there is something you don't understand, you have to ask your employer what it means.

It is probably 3rd party sick pay and is reported there only for information.

If you see this in box 14, I am guessing that this is some sick pay from some insurance company, where the premium is paid by your employer. So, the W2 needs to be reported.