One liter is equal to 34 ounces of water. 3.6 ounces of DME is roughly 0.63 cups. That would be about a half cup with a little mound on it. I would spoon the DME into the half cup measure and then lightly tap the side of the cup to settle the DME. Then add the small mound.
You don't need to boil the water with the DME for 15 minutes. Have the water at a light boil for about 3 to 5 minutes to sanitize the pot. Remove from heat and stir in the DME to dissolve. Recover the pot to keep the mixture sanitary.
It would help out a lot if you used a scale. A $20 Walmart scale works well. Digital scales usually read in pounds and ounces, ounces, and grams. The gram readout is great for measuring DME and splitting a package of hops. (Scale can do regular kitchen duties also.)