Hi

Marks obtained by you in neet is valid for 3 years but only for taking admission MBBS abroad and not for taking admission in India.

So, if you had passed your Neet in 2018 then your marks was valid for taking admission in abroad for the year 2018,2019 and 2020

For  admission in India marks obtained by you in neet is valid for only one year i.e it is valid only the year in which you attempt the exam.
So, if you had passed your Neet in 2018 then your marks was valid for taking admission in 2018 only in India.

But,
don't worry you can still appear in NEET and get admission in  medical college after qualifying it on the basis of marks or Rank obtained by you in NEET.

This is because,

There is no attempt limit for Neet  examination.

You can check Neet 2021 eligibility criteria provided below

Neet 2021 eligibility  criteria says that, to be eligible  one must be either appearing for the class 12th board examination 2021 or one must have passed 12th board examination with physics, chemistry ,biology/biotechnology and English and must have got:-

-) at least  50  % in aggregate of PCB  i.e Physics, Chemistry and Biology/Biotechnology  in 12th examination  if belongs to general category

-) at least  40%  in aggregate of PCB if belongs to  Obc/ Sc /st category

-)at least 45 % in aggregate of PCB in case of general/ews pwd candidates to be eligible.

Also,
* to be eligible for neet 2021 one has to be at least 17 years old or more by 31st December 2021
* There is no upper age limit for Neet 2021
* There's no limit for the maximum number of attempt/times one can appear in NEET.

For detailed information about neet examination eligibility go through our page for which link is provided below; -

https://medicine.careers360.com/articles/neet-eligibility-criteria

Thank you