3 weeks post op Tummy Tuck, draining yellow discharge from belly button. Could this be an infection? (photos)

The belly button appears somewhat reddened.  Drainage is not particularly normal in this circumstance.  You may have a stitch infection which can be treated with local care and removal of the stitch with or without antibiotics.   Either way, your surgeon needs to see you.

Drainage around the belly button is uncommon but does not usually indicate an infection. The picture you included does not show much redness and the fluid is yellow in color. If you are not running a fever and the abdominal area is not warm to the touch then I doubt you have an infection. The few times this has happened to my patients I have simply put them on some antibiotics and the drainage has disappeared in about a week.

it appears from your photo that you still have stitches in your belly button.  these could be the source of the drainage and redness at the skin edges.  see your plastic surgeon for an in-person exam so they can determine if the stitches are ready to be removed and whether or not there is an infection.  this is something that can often be easily treated in the office with local wound care, but you need an in-person examination for further treatment recommendations and management.