I agree with Blossom and Austinmshauri. Overall, I think these situations can present complex, hard-to-determine situations, but what you’ve described in this case now actually seems clear cut. You all agreed that assets prior to your blended marriage would go to the respective children. The house proceeds belong to the daughter. I think even suggesting otherwise could lead to tremendous bitterness and unhappiness in the family, because it really seems wrong.
Of course as parents we can all relate to your desire to get the best education for your children. But don’t use “whatever means possible” if that means trying to horn in on your stepdaughter’s rightful inheritance. I think you jeopardize the health/happiness of your family. Your triplets will be fine (thrive, I’m sure) at an affordable school. They also have the future benefit of any additional support their biological dad will give them over the years and potentially an inheritance from him down the road. They are certainly old enough to understand that the house proceeds go to the stepdaughter given your agreement, so they should not have any ill-will. And I agree; she earned her merit money and should benefit from that.
Good luck; it does seem hard!!