Assuming you are asking for MBBS then AIIMS application window is open whereas NEET forms can be out anytime and JIPMER will be releasing the forms on 7th-March 2018.

The important dates are given in below links -

https://medicine.careers360.com/articles/neet-important-dates

https://medicine.careers360.com/articles/aiims-mbbs-important-dates

https://medicine.careers360.com/articles/jipmer-mbbs-important-dates

Good Luck!!!!