By default, resources are assigned at their maximum units to the task. So a two day duration task and a resource assigned at 100% (default value) will result in 16 hours of work. If you want to allow the resource two days (duration) to accomplish 3 hours of work, you'll need to initially set the task type as Fixed Duration. Enter the 2 days duration and then assign the resource with 3 hours of work. Project will calculate the Peak units necessary to spread the 3 hours of work out over 2 days. Then you may want to set the task type back to either Fixed Units or Fixed work.