Hello Aspirant! If you're asking about JEE Mains and Advanced then Yes it's okay to write 3.00 as 3 as your answer. It will be treated as a correct answer and you will definitely get the marks. If the number after the decimal was something other than zero, then you wouldn't be getting marks but as it's zero it's perfectly fine. No need to worry.
Hope this helps.

Hello Aspirant,

Yes definitely you will get marks because you have submitted the correct answer and if the answer is 3.00 but you wrote 3 then it is alright because they both are similar figures, so you will definitely be awarded full marks for this answer.

I hope this information helps you.

Good Luck!!