Hello Manish,
Actually the gap between 10th and 12th does not effect your admission in iit.If you repeat 11th twice,that doesn't matter but if you repeat 12th more than twice,then you may not get oppourtunity to get into iit.Since you can write jee mains thrice and jee advanced twice,they consider either 12th marks (ongoing) or which is already done(a year back).So your gap between 10th and 12th doesnt matter,but difference between 12th and jee matters.
Feel free to ask if you still have doubt.
All the best
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