While your three tiny captives are sleeping, you take some time to prepare your tools, laying them out on a small tarp. Among them are scalpels, bone saws, syringes (filled with morphine, blood plasma and a few other fluids you may need), ropes, thread, needles, anesthetic gas, a bucket to catch blood in, bleach, and a smock so you don't have to worry about getting blood, urine or tears on you while you operate.
When you feel like you're all prepared, you take a small bullhorn out of your bag and blow it, waking up all three fillies, and giving them quite a scare in the process.
"Now girls, before you ask," You giggle, not like they would be talking, they're all gagged anyway, "I'm going to have some fun with you. Horrible, horrible fun."
You grab one of the scalpels between your teeth, and walk over to Diamond Tiara. You carefully slide the blade down the bottom of her hoof, making a little gash in the soft flesh. She starts crying at the sight of the blood.
"Tsk, tsk tsk," You say, "You won't last very long if you can't stand that. You had better bring all your courage, little missy," You turn to the other two, "you girls too. This is going to hurt a lot. A whole lot. But I'm going to need your support on the matter, even if you don't want to give it to me. We're going to make some great advances in science, you know that?"
The girls have no idea what you have planned, and their little imaginations are probably what's scaring them the most. Though, they should be just a little scared, because today's experiment is going to be...