Connect ec2 instance directly to ecs cluster

I am trying to run 3 ecs tasks on 3 seperate ec2 instances each with their own docker image within 1 ecs cluster. The docker image calls upon a local config file that aims to allow the ec2 instances to communicate/interact with each other via the pre set elastic IP addresses associated with each ec2 instance. Right now I am unable to specify which cluster to directly connect each ec2 instances so I can then run the specified tasks. Additionally, i am not sure how to assign the task onto 1 specific instance within the group.