I am preparing for jee mains exam 2018 as my third attempt. If joint entrance exam changes to single entrance exam then will I be eligible to appear that single exam. As being a third time repeater I am eligible only for mains not for advance .

dear apoorva 
no issue , in case any changes will be their , new term and condition elligibility will comes out for that . 
so, its difficult to say anything about that . 
so, as of now you are elligible . so, start preparation . 
but this is totally a loss of time , 
THERE re no guarentee that u will qualify jee in 3^rd attempt also , then you have to again take that simple college for btech and that will create issue in placement 
HOPE IT CLEARS 
regards 
kindly approve