Adafruit USB LiIon/LiPoly charger - v1.2, charging multiple batteries with series output WITHOUT switch?
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. 3-pole relay solutions.

You may find it easier to use readily available components. I haven't checked, but I suspect that the more poles are added to the relay then the stronger the actuator has to be and the more current required. This would have the effect of (kind-of) making the current required by the relay coil proportional to the number of contacts so there wouldn't be a hugh penalty for additional relays.