Why some say the Canon has the best image quality.

All three cameras use the same size sensor assembly.
The Sony has 20 MP (megapixels).
Panasonic has18 MP.
Canon has 12 MP.

Putting a large number of pixels on such a small sensor assembly means that each pixel must be smaller.
Smaller sensors mean more noise is generated.
Crowding so many pixels together causes crosstalk which results in additional noise.

The Canon has the least MP but the largest size pixels and the least crowding.
You get less noise and more detail.

To get rid of noise the camera maker must use noise reduction firmware in the camera.
Noise reduction firmware will also remove fine detail along with the noise.

When viewing the Canon photo at full size (called pixel peeping) you will see more detail.
On the other two cameras you will see the noise reduction problem on things like blades of grass and leaves on trees.
Instead of clear edges of the individual blades of grass and/or leaves, you will see a blending of the individual grass/leaves. It leaves a mushy look.

If you print your pictures at 4 x 6 inches you will never see the problem.
If you print at 10 x 13 inches or larger the Canon print will look better.

All of the cameras can produce excellent (daylight) photographs.
But they will struggle with photos with dim lighting.
Dim lighting means the camera must use higher ISO settings, and that usually means more noise.
Just keep the ISO setting under 400 and the noise level is usually OK.
For darker shots use a tripod and you can use longer exposure times.