Answer:
Nothing is happening.
Step-by-step explanation:
As written, nothing is going on. H2+O2+H2O represents a mixture of H2, O2, and H2O. We aren't even given the states, so they may all be gases, liquids/solids, or dissolved gases in a liquid (water).
If we had H2+O2 → H2O, we could say that hydrogen and oxygen are combining to form H2O, water. We should note, however, that the chemical equation is not balanced. There are two oxygen atoms on the reactant side, but only one on the product side. A balanced equation would read:
2H2 + O2 → 2H2O
It would be nice to indicate the physical states, such as:
2H2(g) + O2(g) → 2H2O(l)
Two gases, oxygen and hydrogen, combine to form liquid water.
Also missing from this equation is the energy that may be consumed, or released in this reaction. It would be nice to know, for example, that this reaction releases a lot of energy. Otherwise, we might wind up in the local headlines.