Hello aspirant ,
Yes you can apply for Cat 2019 during your third year . But the point to be focused over here is its score is only valid for one year . So its would be of no use without completing graduation.  If you giving it for experience basis or to know exam patter and difficulty . Ya you can surely go for it. Better you focus on you present graduation as well as it is going to be key factor deciding you career.
Good luck
Hope it helps.

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