Hello swaroop !! No,you cannot give GATE examination in third year of your graduation ,you  must complete the final year .But ,you can appear for gate while pursuing final year .Eligibility is to pass your graduation ,there is no requirement of minimum pass percentage ,no age limit and no limits for Gate applicants  you can apply end number of times.You just go through the link provided below to get more information.

https://engineering.careers360.com/articles/gate-eligibility-criteria

Hope this would help you.All the best !!