Here's the relevant ability from Xanathar's guide to Everything:
Prepare Meals. As part of a short rest, you can prepare a tasty meal that helps your companions regain their strength. You and up to five creatures of your choice regain 1 extra hit point per Hit Die spent during a short rest, provided you have access to your cook's utensils and sufficient food.
This ability comes as part of proficiency with cook's utensils.
Does this ability stack? Is there anything in the rules that would prevent it from stacking? I have a group where we allow custom backgrounds (per the rules in PHB), and three players out of the five independently took cook's utensils. I'm wondering if they all get +3 healing per Hit Die.