How do we know which chemical comes after mixing 3 or more chemical, on paper work like balance equation, example : H2 + NaCl + So4 ~ ? What is the product how we can know
Chemical equations follow experiment. Experiment does not follow chemical equations.
So how do you know? You do the experiment, and interpret the result. You know that there is a mass balance. If there are 10 g of reactant, there can be at most 10 g of product. In practice, you are not even going to get 10 g. There is also a balance in electronic charge; there does not seem to be one on your equation.
So to your question; you mixed dihydrogen gas, sodium chloride, and sulfate dianion (i.e.