It’s most likely just a phase. Keep doing what you’re doing, maybe take him to a Dr for a check up, he’ll grow out of it

i would go to your doctor and see if he needs supplementing, my son who is now 6 went through the same problem and only now eats veggies properly. definitely cutting out junk food and i use to make muffins made from pumpkin and grate finely veggies in them. i think the more healthy foods you give him the less he will resist it may take a few weeks of presenting him with the food but persistence is the key (even though its so hard, stick with it)

Chop them up really small using a food processor, thermomix or tupperware extra chef and hide them in sauces, meatballs etc

Does he like meatballs or sausage rolls? If so finely grate the veggies and mix it in the meat.

Do you eat with him? my son is much more likely to eat foods if he sees us eating it too. But in saying that it is quite normal for 3yr olds to be picky, aslong as he is getting some vegetables and you always offer vegetables I wouldn’t worry too much. Good luck. x

Sometimes the sugar and salt and fat in junk food can alter the taste buds to make vegetables almost unpalatable, so reducing (or eliminating) any highly salty, sugary or fatty foods would probably help him adjust to vegetables. If he wont eat them when “hidden” in recipes (eg grated carrot in mincemeat or grated zuchinni in brownies) erhaps you could try roasting some vegetables since this makes them sweeter. Chop virtually any hard vegetable (carrots, sweet potatoe, pumpkin are nice and sweet to start with), lightly coat in olive oil and bake in over for 50-60 minutes or until soft, roasted and slightly caramelized. he might like those. Good luck.

My son is a bit younger, but I have always found mashed veggies with a bit of butter. Or sometimes I found a bit of Apple sauce on top helps

Have you thought about hiding veggies in your pancakes or brownies ?
I work a lot with lentil flour and besan flour. Lentil flour is made of 100% lentils and besan flour is made of 100% chick peas. Both are perfect for pancake batter or for whatever you like to bake.
I often add zucchini, beetroot, sweet potato or beans to my brownies. When you grate them fine (by hand / blender) you won’t find them back in the end product.