Can we change from mtech to btech in the same branch after 3rd year?

Mrinal, your query is little ambigiuos. It is not clear what you are doing. Most of the universities offer two types of degrees- Integrated and Dual. In Integrated degree, you cannot opt out but in Dual degree you can opt out after lower degree. 

One of my distant cousins is studying in Lovely Professional University. He is doing Dual B.Tech-M.Tech and in his third year (last part of the third year i.e 6^th semester), he was asked for option whether he wanted to continue with his M.Tech or not.

Rest you can check with various other universities.

Best of luck.