Project can only schedule work for resources during working time. If you have a 3 day duration task originally scheduled to start on Monday and assign a resource (R1) who doesn't have Monday as a working day, the task will be rescheduled for Tuesday. If you then assign another resource (R2) who works on Monday but has Tuesday off, the task start will revert to Monday and the duration will increase to 4 days.

R1 works 25.5 hours on Tuesday, Wednesday, and Thursday.
R2 works 25.5 hours on Monday. Wednesday and Thursday.

If you then assign a resource who doesn't work on Wednesdays, Project will calculate 34 hours of work (4 days duration * 100%) scheduled for Monday, Tuesday, Thursday, and Friday and increase the duration of the task to 5 days.

So, the total amount of work on the task is now 85 hours. Did you mean it to be 25.5 hours? If so, the task needs to be effort driven and that will certainly change the duration.

R1 (Not working on Monday) assigned will calculate the 25.5 hours of work scheduled for Tuesday through Thursday. (8.5 hours per day)
Assign R2 (doesn't work on Tuesday) and the 25.5 hours of work will be split evenly - 125 hour each. The task start will start on Monday and end on Wednesday morning with the following work schedule:
R1 working 8.5 hours on Tuesday and 4.25 hours on Wednesday
R2 working 8.5 hours on Monday and 4.25 hours on Wednesday.

Assign R3 (doesn't work on Wednesdays). The 25.5 hours of work is now divided between the three resources however, not evenly. The task ends Tuesday afternoon.

R1 work 6.95 hours on Tuesday
R2 works 6.95 hour on Monday
R3 works 8.5 hours on Monday and 3.1 hours on Tuesday.

The task duration is now 1.82 days.

I hope this helps.