How to extract a column that has the highest value within row in Hive?
I can't test in hive, but a possible SQL query is as follows (greatest returns the maximum value from the list):

select 
  case 
   when col1 = greatest(col1,col2,col3) then 'col1' 
   when col2 = greatest(col1,col2,col3) then 'col2'  
   when col3 = greatest(col1,col2,col3) then 'col3'    
  end as c1 
from test;


Additional note:
you should check, how ties are to be handled, in my solution I simple take the first column.