Mix the milk and water and heat the mixture until it reaches 105°F. Stir in the active dry yeast and let it sit for 10 minutes. Then knead the flour with salt, sugar, room temperature butter, and the yeast mixture for about 15 minutes until a smooth, supple dough starts to form. Cover the dough with plastic wrap and refrigerate for 12 hours.
Roll out the 3 sticks of butter between two pieces of parchment paper into a rectangular shape. Chill the flattened butter in the fridge for about 15 minutes.
Take the dough out of the fridge and roll it out into a rectangle.
Place the flattened butter in the middle of the dough.
Wrap the dough around the butter.
Roll out the dough into a rectangle. Fold the dough in the middle. Fold the rest of the dough onto the dough that's been folded out to form a rectangle. Cover the dough and then put it in the fridge for 30 minutes.
Roll out the dough into a rectangle again. Fold both sides in the middle. Fold the dough again, cover it, and put it back in the fridge for 30 minutes. Then roll out the dough again and repeat the process of folding, chilling, and rolling out the dough 3 times. Wrap the dough in plastic wrap and refrigerate for 12 hours.
For the poppy seed filling, boil the milk, sugar, and vanilla extract. Then stir in the poppy seeds and semolina flour and let it boil down, stirring constantly. Allow the poppy seed mixture to cool down once it's done. For the raspberry filling, mix all the ingredients together and mash them with a fork until you get a smooth mass. For the blueberry filling, add all the ingredients to a saucepan, boil until the mixture thickens, and let cool.
Roll the dough into a rectangular shape and use a knife to straighten the edges if necessary. Cut the dough into 3 equally wide strips. Cover each strip of dough with a filling and carefully roll it up.
Place the rolls next to each other into a loaf pan lined with parchment paper, cover the pan, and let the dough rise for 2 hours at room temperature.
Whisk an egg and brush it over the croissant loaf. Place the loaf pan in a preheated oven set to 375°F and bake for 45 minutes.