Example 1
Given that I found a severed thumb in my roasted red pepper soup, no, this meal was not altogether satisfying.
We hope the speaker got a free dessert out of this culinary misadventure, preferably at a different restaurant. Here, altogether is the correct word because it means completely.
Example 2
"If we want Mr. DeVlorpe to curve our grades on this algebra test," Drew explained to his classmates, "we should present our reasons to him all together."
Safety in numbers, Drew. Good plan. In this example, all together is the way to go because it means as a group.
Example 3
The kindergarten band concert was painful. Those pint-sized musicians couldn't start a song all together if you paid them.
Well, that's harsh. In this sentence, all together is the correct word because it means in unison, which is a state that's virtually impossible for a bunch of five-year-olds to achieve.