I am unable to join tables in correct manner
Try this out,

This should work, or at least get you on the right path of getting the results you are looking for... pay attention to how the tables are alias and how the joins are used as that is key for getting the proper results...

1)
SELECT P.PID
     , CONTACT(R.FNAME,' ',R.LNAME) AS FULL_NAME
     , P.PCONTENT
FROM REGISTRATION R
JOIN POST ON R.RID = P.RID


2)
SELECT P.PID
     , P.PCONTENT
     , COUNT(PL.PID) AS TOTAL_LIKES
FROM POST P
JOIN PLIKESTATS PL
ON P.PID = PL.PID
WHERE PL.LIKESTATUS = 1
GROUP BY P.PID
       , P.PCONTENT



3)
SELECT P.PID
     , P.PCONTENT
     , COUNT(C.PID) AS COMMENT_COUNT
FROM POST P
JOIN COMMENT C ON P.PID = C.PID
GROUP BY P.PID
       , P.PCONTENT