About 4 cups per L, about 1 cup DME per L, so about 3 cups DME and 12 cups water, yup.
If stepping up, you usually do it more geometrically...so yeah, you could make a 2 cups water starter with 1/2 cup DME, then add 10 cups more water and 2.5 cups more DME to bring it to 3L.
Weighing is better, but who is to say that 1.037 OG starter is any better/worse than 1.045 OG starter? Just eyeball it if you don't have a scale. If you DO have a scale, it's 11 grams DME to 100 mL water for 1.040 OG. For 3L this is 330 grams DME.