Having read and considered sources A, B, and C, can you infer something else - D (not a source, but your own idea)?
Because a synthesis is based on two or more sources, you will need to be selective when choosing information from each.
If you imagine a synthesis essay as a room in which the synthesis writer is joined by the authors of her/his sources, the 4.0, 3.5, or 3.0 essay has everyone engaged in conversation or debate, with everyone commenting on (or arguing against) each other's ideas directly.
In the 2.5 and below essay, each person in the room stands up in turn, gives a speech, and sits down, with little or no question and answer period in between or afterward.4.
Your purpose in writing (based on your assignment) will determine how you relate your source materials to one another.
Your purpose in writing determines which sources you use, which parts of them you use, at which points in your essay you use them, and in what manner you relate them to one another.
Is the information in source B, for example, an extended illustration of the generalizations in source A?
Would it be useful to compare and contrast source C with source B?
Some relationships among the material in you sources must make them worth sythesizing.
It follows that the better able you are to discover such relationships, the better able you will be to use your sources in writing syntheses.