Panasonic ZS3 has a 10.0MP 1/2.3-inch (6.08 x 4.56 mm ) sized CCD sensor . On the other hand, Panasonic ZS40 has a 18.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CMOS sensor and features Venus Engine processor.
Panasonic ZS40's sensor provides 8MP more than Panasonic ZS3's sensor, which gives a significant advantage in real life. You can print your images larger or crop more freely.
On the other hand, please keep in mind that Max sensor resolution is not the only determinant of resolving power. Factors such as the optical elements, low pass filter, pixel size and sensor technology also affects the final resolution of the captured image.
Below you can see the ZS3 and ZS40 sensor size comparison.
![Panasonic ZS3 vs Panasonic ZS40 Sensor Size Comparison]()
Sensor Size and Resolution Comparison image of Panasonic ZS3 and Panasonic ZS40 Cameras
Panasonic ZS3 and Panasonic ZS40 have almost the same sensor size so none of them has any significant advantage over other in providing control over depth of field when used with same focal length and aperture.