I assume that HR table is the parent table, and that not all HR is in EMployee AND Student.
Without more information and data to validate, my guess would be something like this.

SELECT
  h.HRID
, h.VaFrmDate
, h.VaToDate
, e.RecDate
, s.SevDate
FROM HR AS h
LEFT JOIN EMployee AS e
 ON h.HRID = e.EmpID
AND e.RecDate >= h.VaFrmDate
AND e.RecDate <= h.VaToDate
LEFT JOIN Student AS s
 ON s.StdID = e.EmpID
AND s.SevDate >= h.VaFrmDate
AND s.SevDate <= h.VaToDate;