19 months old is actually depending more on solids to make them full. So, I guess the 3 hours continuous latching is due to comfort latching. What you can do include: delay bedtime since he is not sleepy yet, distract him by bringing him to outdoor for a walk, offer additional snacks if he seems very hungry after dinner (finger foods, fruits, and etc.). If you feel your nipples are sore, try to apply some nipple cream to soothe the nipple.
19 months reply alot on solid food rather than milk. The 3 hrs latching is most like comfort latching. Even my 7 months old baby loves comfort latching as it gives them a sense of security. If he is a big size toddler, then it will easily explain his intake. Another is that if he is active. They are growing up and will eat and drink lots. It is normal to me.
Hmmm is your child a very active child? If he is, this could explain the amount of food he is eating or that he has a very healthy appetite. Size of the baby also contributes to how much he eats -- my cousin's baby is biracial (half caucasian) and is a pretty big baby. He is nearly 2 years old and I notice he eats and drinks alot too.