You must have any single reservation under the same dining plan. You'd need to upgrade the whole thing to basic dining or deluxe dining, or leave the whole thing QS.
In order to have two different dining plans, you'd have to book two different packages. Which, of course, doesn't make sense because you'd lose a ton on tickets, and other costs. Negates any savings.
If all you want are 3 TS meals, I'd just pay for those out of pocket.
Do all the math though, and make sure you are saving money. By paying premium prices for tickets, rack rate for the room, and lots of out of pocket costs on dining, you might be spending more than you think.