Are you starting these services manually in the console?, If so there is a section in the "create" under Services tab in the Cluster UI called "Task Placement".
By default I see that a template is selected being "AZ balanced spread" this template uses the "Spread" placement strategy, that tries to spread the tasks amongs AZ. You can try the "binpack"* strategy, this strategy tries to maximize the use of ec2 resources.
*Tasks are placed on container instances so as to leave the least amount of unused CPU or memory. This strategy minimizes the number of container instances in use.
More about placement strategy