If 43%=0.43 applies evenly to the whole month then it applies to each week within the month. To work out the probability of rain 3 days out of 7 in each week, when there is a probability of 0.43 of rain on any particular day and a probability of 1-0.43=0.57 that it won't rain, we need to find out how many ways can we pick 3 days out of 7. This is given by the fraction (7*6*5)/(1*2*3)=35. This is the nCr combination function, where n=7 and r=3. It is derived from the way objects can be selected: we have 7 objects (days) and we pick one, leaving 6, so we pick another, leaving 5, and we pick the third out of the remaing 5. This gives us 7*6*5=210 ways of picking in order three objects out of seven. But we're not concerned about the order, so we need to divide this by the number of ways we can arrange 3 objects, that is, 6; 210/6=35. If we want know the probability of rain 3 days in the week then we multiply (0.43^3)*(0.57^4) by 35, because there's a probability of 0.43*0.43*0.43 that it will rain for 3 days and a probability of 0.57*0.57*0.57*0.57 that it will not rain for 4 days. The product is 0.2937 or 29.37% probability that it will rain 3 days a week.
