If the number of guests at Betty's party will now be 125% of the original number she planned, then the number of guests is 1.25 times the original number. Let's call the original number of guests "x". Then the number of guests at the party is 1.25x.
If Betty needs to increase the amount of ingredients in all the recipes, including a recipe that calls for 3 cups of flour, then she needs to increase the amount of flour by the same factor of 1.25.
The amount of flour she needs now is:
3 cups x 1.25 = 3.75 cups
Therefore, Betty will need 3.75 cups of flour.