Extand main APFS partition on MacOS Mojave 10.14.3
The output, from the commands below, should show that disk0s3 starts at an offset that is sufficiently close to the end of disk0s2 to produce the error message you posted.

diskutil info disk0s2 | grep -e Offset -e "Disk Size"
diskutil info disk0s3 | grep -e Offset -e "Disk Size"


Or, in other words:

(Offset of disk0s2) + (Size of disk0s2) - (Offset of disk0s3) = (Small enough size to cause message)


You do not need the Apple_KernelCoreDump partition. You can remove this partition be entering the command below.

sudo diskutil eraseVolume "Free Space" none disk0s3


Next, you can reenter the command you posted in your question to reclaim the free space.

diskutil apfs resizeContainer disk0s2 0