Count missing values across columns and join back to original table
SELECT id, date, t1, t2, t3
     CASE WHEN t1 IS NULL THEN 1 ELSE 0 END 
     + CASE WHEN t2 IS NULL THEN 1 ELSE 0 END 
     + CASE WHEN t3 IS NULL THEN 1 ELSE 0 END AS missing_across_all_cols
   FROM testdata