I do it the old fashion way, as an arithmetic problem. I use the packaging and Calorie King. A cup of all purpose flour is 88 carbs, a banana is 39 carbs (3= 117) and 3/4 cup of sugar is 150. Did you add milk or other things? If not, the total amount of carbs in the proportions you gave is 399. If you made 12 muffins, each would be around 34 carbs.
I do this a lot, as I often make bread. I can calculate the total carbs and divide by servings and get pretty accurate numbers. Just watch out for the fat. It won't add carbs, but can affect your absorption rates.