Preheat oven to 375 and line 2 large baking sheets with parchment.
Whisk the flour, baking soda, and salt together in a bowl and set aside.
Zest the 3 grapefruit and add it to your mixer bowl with the sugar. Using your mixer, a spoon, or your fingers, really work the zest into the sugar for a couple of minutes.
Remove ½ cup of the grapefruit sugar and place onto a small shallow dish.
Add the butter to the remaining sugar and cream for 3-4 minutes until light and fluffy.
Juice ½ of the grapefruit and reduce the juice in a small saucepan over high heat to 1 Tbsp of liquid.
Add the grapefruit reduction to the butter mixture and mix.
Scrape down the bowl and add the two eggs until well blended.
Add the dry ingredients and mix for 30-60 seconds on medium-high.
Scoop the dough into balls and drop into the reserved grapefruit sugar, rolling and tapping off any excess.
Place 8 cookies on each lined baking sheet and refrigerate the dough balls for 15 minutes.
Bake for 10 minutes - the edges should be light brown and the centers will look pale and barely set.
Leave the cookies on the pan to cool for 5 minutes and then transfer to a rack to cool completely.
Store cookies in zipper bags for 5-7 days.