Your at wills represent your core competencies, and must be from your main class, unless you go with paragon multiclassing. 4e is intended to be a largely single-class system, and to be a "true" multiclass character (that is, a character with core competencies in two different classes), you need to do it paragon-style.
The argument for paragon-multiclassing is that the added versatility you gain from your second class's powers makes up for the 11,11,16 bonuses you lose and the fact that the powers you gain are slightly lower level than those you would have normally. I have no problem with this option, but it does not appeal to everyone's tastes.
As to the human question, I would say no. If you wanted two of your 3 at-wills to be from a different class than your main class, then you should select that class as your main class and multiclass into your original class.