I wouldn't say that. It sounds as if he bought a can containing just three lentils! As a rule, with tinned/canned food, we just give the number of tins/cans, then add "of" and the name of the food.
I need three tins of tomatoes.
Can you buy me four cans of lentil soup?
If you can't find me two tins of tofu, get me a couple of tins of seitan instead.
Without numbers or "tins of", you can use "tinned/canned + name of food".
I hate tinned soup.
My son can't stand canned beans.
Her cousin won't eat anything except tinned jackfruit.