In a medium saucepan over medium heat add butter, milk, water, sugar and salt to the saucepan.
When the butter is full melted, let it come to a boil. Reduce heat to medium low and add the bread flour.
Mix the bread flour well to let it combine into a ball using a wooden spoon. Keep mixing it for the next 3-4 minutes. The ball should come away from the sides of the saucepan with ease.
Remove the saucepan from the heat and transfer the éclair pastry dough to a stand mixer fitted with the paddle attachment. Mix the dough in the standmixer on low speed for 3 minutes.
Add an egg and mix until fully combined. Add the other 2 eggs the same way until it's fully combined.
If the dough is wet, soft, and has an even consistent surface, you don't need to add another egg, your dough is finished. If the dough is a bit on the drier side, add another egg.
Fit a piping bag with a 1/2" round or star tip. Add eclair pastry dough to the piping bag.
Pipe eclairs onto a silicone baking mat. They should be about 4" long and 3/4" in thickness. Space the eclairs 1.5-2" apart from one another so they have enough room to puff up in the oven.
Bake at 425 degrees F for 10 minutes. Reduce the heat to 325 degrees F and bake for 30 minutes. The eclairs should be a medium brown color.
As soon as the eclairs come out of the oven, poke them 2 times each with a toothpick. This lets the steam out of the eclairs so they will maintain their shape and not deflate from the trapped steam inside.
Cool the eclairs on a wire rack.