I'm pursuing my CSE degree from BE and now I'm in 3rd-year & was thinking to apply for civil branch in polytechnic, is it valid?

At this stage, it will not be beneficial to change your mind and go for Diploma as you have been invested your precious year in B.E. I will suggest you to go with same stream otherwise you can do M.Tech in civil engineering by qualifying GATE exam which will be more fruitful for you.

Why will you even want to go for Polytechnic when you have got a good branch in BE? Polytechnic and BE has a lot of difference. Generally, it is seen that people after Polytechnic come for B.Tech but vice versa is not true. You can do one thing. Prepare for GATE in civil branch. Practice civil paper