Converting selected ec2 on-demand instance to reserved of same type

That's not exactly how Reserved Instances work. You can't "convert" your on-demand instance into reserved.

Reserved Instances are a billing construct. It's a commitment to use certain types of resources continuously on an hourly basis for a whole term (i.e. 12 months or 36 months). The discount you get from your RIs depends on multiple factors, some of which are attributes of your RI provided that it is being utilized by your running instances.

When you made a commitment (purchase RI) - the discount can go to ANY of your running EC2 instances that match the attributes of your purchased RI. The discount will apply automatically as long as there is matching usage. SO, instead of paying on-demand fee, your running EC2 instance will be charged $0 during the hours when they will be covered by RIs, and you will only end up paying for your RI commitment term.

You cannot specify to which running instance you want your RI discount to apply. Since RIs are a billing mechanism - the discount will simply apply to matching usage. And the discount may be applying to a different instance (out of your three same instances) during different hour. The discount mechanism works in such a way to ensure you get all the benefit. Imagince if you stop or terminate one of the instances - then the discount will automatically rollover to the next matching instance.

Coverage for reserved instances varies depending on the method of purchase.
For example, if all three EC2 servers are located in different availability zones and you purchase zone-reserved instances, you can apply them to only one of them.
For region-reserved instances, all EC2s that match the criteria are eligible.
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/reserved-instances-scope.html
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/reserved-instances-types.html