Take the percentage of it not raining over all the 3 days and times them together to get the chance it will not rain over all those days.
(0.6 * 0.5 * 0.2) = P of rain over all the days
0.06 or 6 percent that it will rain over all those days.
Taking the chance of it raining Monday is:
100 - 50 = 50 percent chance over monday.
Taking the percent chance of it raining over monay and tuesday:
0.6 * 0.5 = 30 percent chance it will rain over those two days.
Taking the percent chance of it raining over all the days is:
0.6 * 0.5 * 0.2 = 0.06 so a 6 percent chance over all the three days.
Does this make since?