Position your oven rack to the center of the oven and preheat the oven to 425 degrees. Grease and flour or line a muffin tin.
Cut the rhubarb into roughly 1/4" pieces.
In a large bowl, combine the flour, sugar, baking powder, cinnamon, baking soda, and salt. Stir the dry ingredients to combine.
In a medium sized bowl whisk together the greek yogurt, melted coconut oil, eggs and vanilla until smooth. The coconut oil becomes a solid when cool, so if it clumps a bit just keep mixing until the clumps are incorporated or pretty small.
Fold the egg mixture into the dry ingredients with a spatula until they are just combined, be careful not to over mix.
Gently stir in the rhubarb.
Divide the batter into twelve muffin cups, making sure to divide the batter as evenly as possible.
For the topping, combine the 3 tablespoons of sugar and ½ teaspoon of cinnamon in a small bowl and mix with a fork. Sprinkle a generous amount over each of the muffins, about ½ teaspoon or so.
Bake the muffins at 425 degrees for 5 minutes and then reduce the oven heat to 375 degrees and continue to bake for 14-17 minutes. The muffins should be golden brown and a toothpick inserted in the center should come out clean. Let the muffins cool in the pan for 5-10 minutes and then carefully lift them out of the pan and onto a cooling rack.