Apparently MSP divides the workload prior to assigning the schedule. In the resource form or resource usage, you’ll notice that both Resource 1 and 2 will work 9.6 hrs on this project, whereas resource 3 will work 4.8 hrs. Therefore R1 and R2 will work on the task on 7th, 8th and 9th (4+4+1.6 hrs) of January. R3 will work on 7th, 12th and 13th (2+2+0.8 hrs) of January. Which is indeed a strange logic.
As a test I have added another identical task with the same resources on the same starting date. When auto scheduling this task, I’ve noticed MSP now assigns 8 hrs to each resource, even to R3. This next task has an even longer duration, because the 8 hrs workload for R3 take 4 days. This logic is different from the first task, which makes it even more confusing.
As you cannot influence the scheduling logic (as far as I know) manual changes in the resource usage is your only option. Change the work for Resource 1 and 2 to 11 hrs and for resource 3 to 2 hrs.