We have 2 reserved c3.xlarge instance in our project but have total 3 launched c3.xlarge instance.
How to check which 2 instances are reserved out of 3. I checked details of all 3 instances, but nothing is mentioned.
Reserved or not reserved is just a billing convenience. There is no concept of a particular instance being reserved or not.
So for example if you have two instances and have purchased on reserved instance and one is 'on demand', you can delete either one of them, and one of them is still reserved.