If I have done my class12th in 2018 with PCM then can I give exams like jee-main, MHT-CET, BITSAT,UPSEE in 2021?
You can't also appear for BITSAT cause the eligibility criteria of BITSAT 2021 says that the candidate must has passed 10+2 in 2020 or will be appearing for their final examination in 2021. As you have passed 12th in 2018, you can't sit for BITSAT 2021.
And now UPSEE an MHTCET both are state level entrance examinations where candidates have to show their domicile certificate. Either candidates have to belong to the state or their parents. There is no year of qualifying exam criteria for UPSEE and MHTCET but have minimum marks criteria. For UPSEE, a general category candidate needs at least 45% marks for B.Tech and in MHTCET it is 50%. You can check the domicile criteria here http://engineering.careers360.com/articles/upsee-eligibility-criteria
Yes you can give exams like jee-main , UPSEE, BITSAT after passing 12 th in 2018.
Like jee-main you can appear the Jee-main exam for 6 times. Even you can also give 2 times in a year January and April. After completing 12 th you can also give the other exams like UPSEE several times there is no age limit. As you said you had pcm so you can apply all the exams above because all are engineering exams and they asked pcm in 12.
I hope it helps you
All of the exams you have mentioned are engineering entrance exams, and the minimum eligibility criteria to appear for any of these entrance exam is that the candidate must have Physics, Chemistry and Mathematics in class 12th, candidate must have passed class 12th with PCM or may be appearing in class 12th With PCM, so if you have passed class 12th with PCM then you can apply for any of these exams.
Hope this helps, all the best.