I'm guessing you are using Fixed Units, Effort driven tasks. Each resource is allocated a portion of the total work based upon their assignment units and the total assignment units for the task. Both R1 and R2 are allocated 40% of the total work or 9.6 hours each. Their assignment units are 50% of a total of 125% of resource units. R3 is allocated 20% of the total work or 4.8 hours.

If you look at the task usage view you'll see the following distribution of work:

Jan 7
R1 = 4
R2 = 4
R3 = 2

Jan 8
R1 = 4
R2 = 4
R3 = 0 because of the nonworking time

Jan 9
R1 = 1.6 hours
R2 = 1.6 hours
R3 = 0 because of the nonworking time

Both R1 and R2 are finished with their portion of the task on Jan 9 at 11:12 am. R 3 still has an additional 2.8 hours of the original 4.8 hour of work. Because the resource is on vacation, that remaining work is spread out into Jan 12 (2 hours) and Jan 13 .8 of an hour.

If you don't want project to spread the work based upon assignment units you can edit the work. Knowing the R3 is on vacation, I'd say s/he should only get 2 hours of work - leaving the remaining 22 hours split between R1 and R2. That edit drops the duration to 2.75 days and the task ends at 3:00 pm on Jan 9.