The probability of a girl for any birth is 0.5. Here are the ways to get exactly 2 girls
GGB: p = (1/2)^3 = 1/8
BGG : P = 1/8
GBG: p = 1/8
Total probaility = 3?8
All three possibiliites above meet the condition of at least one girl so we can include their probablitites. There is for more possibilities to meet the condition (at least one girl): They are
All of these have 1/8 probabilities so the total probablility is 7/8. Another way to say this is what is the probability of no girls
BBB and that is 1/8. Any other outcome meets the (at least one girl criteria so 8/8 - 1/8 = 7/8.