ETA: I just looked back at your prior questions. Is this your grandson you are talking about? 3 years ago, you wrote questions about your infant grandchild. So are the parents as worried as you are? Does the child just not eat for you, the grandmother?
Original post: There's not enough info here. Is your pediatrician desperate and nervous too? Does your child have Failure to Thrive syndrome? Is there a sensory issue or a problem with swallowing? I'm guessing not. Or are you just a nervous parent with a child below the 50% mark?
If your doctor is worried, then you follow that advice.
If your doctor is not worried, then please understand that a 3 year old will not starve if he misses a meal. If he is hungry, he will eat. Do not give in to tantrums.
I had a picky eater with some mild texture issues. I was excellent at hiding nutrition in things like pancakes and French toast, and at making my own chicken nuggets with wheat germ and flax seed. I also give a comprehensive, complete food supplement (not just a multivitamin) that was delicious in milk or yogurt or pudding. It fills in all the gaps.
Stop making mealtime a battleground. Don't buy chips and pretzels and junk breakfast cereals - "We don't have any" is what you say. "I'm having some delicious apples and walnuts and I will share with you." Mealtime is a pleasant and desirable place to be - if he doesn't want it, he can be excused from the table. No begging, no bribing, no showing any anxiety or stress at all.
Use a sectioned plate or even an ice tray - each compartment can hold any of the following: yogurt, cheese cubes, whole wheat pasta, healthy meats and fish, avocado, hardboiled egg, various fruits (berries, melon, banana) and veggies (sweet potatoes, peas, carrots, and so on). Colorful choices and fun tastes/textures with a wide variety. What he does not want goes back in the fridge until later.
A 3 year old doesn't run the show. Make sure his caregivers have absolutely no option to go to cookies and crackers and ice cream.