At the time of campus placement of MCA at NIT Trichy, are companies checked in the 12th percentage also?

It's the case not just at NITT, but everywhere. It entirely depends on the visiting firm and whether they value your school percentages. 

The placement cell will have all of your information entered into their placement portal, and they will be aware of everything, so if you give the firm an erroneous résumé, whether intentionally or unintentionally, they will be notified. Because there are so many students, your college will try to set minimum eligibility standards for them by requesting or something similar. 

To make things clearer, I'll give you an example. 

1. Let's say Amazon Cloud is on the way, they've left the pointer criteria at 6, with no criteria in the 10+2 range.
2. Deloitte arrives on the second day and demands 60+ in 10+2+3.You won't be able to do anything because Deloitte does it all throughout India. Colleges don't matter as per their rules.

However, if a random startup comes along and offers a nice, attractive package and has policy rules similar to the ones listed above. But the placement cell can request them to ease the criteria if students have low scores. It now hinges on the company's reputation. It is applicable to every institute.