Combinations 2, 4, and 5 will result in the project being one day late. Combinations 3,6,7,8 and 9 will result in the project being two days late. As per the formula, the probability of each result for the project will be the sum of the probabilities of the individual combinations of events that cause that result.
Now as far as the third independent company is concerned as per the given information the probabilities of the timeliness of the project are equal for all three events. i.e. the probability of the project being on time is 1/3 or 0.33. The probability of the project being one day late is 1/3 or 0.33 and the probability of the project being two days late is 1/3 or 0.33.
Now when we compare the two choices from the probabilistic viewpoint, we can see that the probability that the project will be on time eitherways ( if we have Everconc and Steelfix as suppliers or if we switch to the third supplier which supplies both concrete and re-bar steel) is 1/3 or 0.33. However the probability that the project will be two days late is higher in the case of multiple suppliers 0.38 as compared to 0.33 in the case of the single supplier. Hence the option of the single supplier is a better one from the probabilistic viewpoint.
When we look at the issue apart from the probabilistic viewpoint we bring into it other factors like risk.