Hello there ,hope you are doing well.Sorry to say but you can't because you need atleast your sixth semester mark list to apply for gate but if you want practice go for ace or Madeeasy test series or previous question papers.
Feel free to ask me in the comment box if you are having any further doubts
hope you find it helpful
No,you can't apply for GATE2020 since you are in 3rd year..Who are in the final years or have passed bachelor degree can apply for GATE..
For more detail visit below link;