Project needs two variables to calculate the third. You assigned a duration of 3 days, which by default a day is 8 hours. So that put the work at 24hrs. Resource has 100% units assigned by default. So when you assigned the resource to that task, project calculated 8 hours a day because the units for that resource was automatically at 100%.
When you went back and changed the work to 12 hours for work, then project had to recalculate how much work each day the resource could do to keep the duration at 3 days.
As long as the assigned work hours for that resource is less than 8 hours for the day then the resource units for each of the tasks can be 100%.
Since project needs 2 of 3 variables to calculate the third, enter the work hours after the duration, then enter the resource.