Yes, you can do this with a resistor divider. We usually think of a resistor divider as multiplying a voltage by some value less than 1, meaning scaling relative to ground. However, a resistor divider can be set up to scale relative to any particular voltage.
In your case, you can scale the voltage about the 3.3 V supply instead of about ground. Overall, you want to scale a 6 V range down to a 3 V range, so the divider scale factor needs to be 1/2, meaning the two resistors must be equal. For example, one 10 kΩ resistor to the 3.3 V supply, another to the ±3 V signal, and the two other ends connected to the A/D input. That will load the signal by the sum of the resistors (20 kΩ) and provide a output impedance for driving the A/D by the parallel combinations of the two resistors (5 kΩ). If 5 kΩ is not low enough to drive your A/D, then lower the two resistor values so that each is twice the required impedance.
Note that this method requires that the 3.3 V supply is well regulated. That would be the case, for example, if it is driven by a linear regulator from a higher voltage.