HI,

SEE nits and iits have no lateral entry or admission process for diploma engineering passed candidates. only students who have cleared the jee main or jee advance exam after completing 12th with satisfying all the marks and age limit criteria. now coming to your case, diploma took 3 years duration, so even if you have completed diploma after 12th you may appear in jee main, bu for jee advance exam you are not eligible. since candidates may appear in jee advance two times starting from passing year of 12th degree and next year. however if you clear jee main exam, you have to start the b.tech course from first years of study and it would take 4 years.

hope it would help.