select tt.id, abs(t1.len-t2.len) df
from
  (select id, min(Num_challenge) min_nc, max(Num_challenge) max_nc
      from t group by id) tt
join t t1 on t1.id=tt.id and t1.Num_challenge=tt.min_nc
join t t2 on t2.id=tt.id and t2.Num_challenge=tt.max_nc
order by df desc
limit 3