Dear Aditya,

Yes, you appear in CLAT for 3 year LLLB course after civil engineering. 

And diploma is not 12th. It's a specialization course in a particular subject. There's an undergraduate diploma ( graduation degree required) and a postgraduate diploma (after 10+2). 

Hope it helps.

Good Luck.

Dear Aspirant,

If you are really interested in studying law and wants to appear CLAT then you have  complete 10 +2 from any stream . Diploma in Engineering is not equivalent to 10+2