I’d say give some guidance and be a good example yourself. Maybe make up some well reasoned rules… McDonalds is okay every now and then if you also eat healthy food every now and then. Make sure he gets some fibers and vitamins and maybe you can get even a small kid to understand that there is a reason behind it. In the end I don’t think you have to apply force… I’ve seen children grow up on mostly plain noodles, water and a few other things and they turned out alright. But also don’t enable such behaviour. I think there are some rough rules of thumb… Like a kid needs to try something 6 to 15 times to like it (at all). Liking food is a lot about what we’re accustomed to, and not just the taste. And the early time in life is important to develop a diverse palette. And the parents are an important role model, so if they don’t have a wide spectrum of food, the kids are also very unlikely to get any different, no matter what you do. I’d say maybe read a (good) book on the subject. This is a fairly common problem with children. (And drinking just water is fine. I’m not sure if that applies to 3 year olds, but in general that’s a healthy thing to do.)

If your kid is special, you might have to try a few things and see what works. But from looking at several specimen, I’ve come to think children are very individual anyways and there’s often not one thing that applies to all of them. Some people have rules. Some make the food look good, maybe carve a crocodile out of a cucumber every now and then. You can manipulate a 3yo and serve them arbitrary food in McDonalds containers if it’s just that. Or have them watch other kids or people and see what they do to give some inspiration…