How the input entered through the front-end form match with the existing data in database
Select diseases with the 3 symptoms:

select DISEASE_ID, count()
  from DiseaseSymptoms_T
 where SYMPTOM_ID in (1, 2, 3)
 group by DISEASE_ID
having count() = 3  -- number of symptoms in the where

That will only list typhoid, because it has all three symptoms

If you want all the diseases with the symptoms 1 and 2

select DISEASE_ID, count()
  from DiseaseSymptoms_T
 where SYMPTOM_ID in (1, 2)
 group by DISEASE_ID
having count() = 2  -- number of symptoms in the where

This last query will also show the disease typhoid, because it has the two symptoms in the where

If you want only the first 5 rows, the syntax depends on the engine. For SQL Server use SELECT TOP 5 DISEASE_ID... For MySQL add LIMIT 1 at the end of the query... For Informix use SELECT FIRST 5...