is there any way to set up a Raspberry Pi 3 Model B+ wirelessly (headless) without editing the microSD card contents to enable SSH?
No. That is not possible. You can't run headless without ssh. You can't enable ssh without adding a /boot/ssh or /boot/ssh.txt (and a /boot/wpa_supplicant.conf file for WiFi) before first boot.
That is all done on the small FAT partition, so is easy to do on any Windows, Mac or Linux system.
To amplify @Dougie's correct answer:
You'll likely be writing your Raspbian Stretch Lite image to your SD card after download. This would be the best time to do this. The
ssh file need not have any contents at all. Depending on the system you've used to download & write the image to your SD card, one of the following will do it:
$null > /boot/ssh
Mac or Linux:
/boot is a
FAT partition, and so can be mounted by Windows, Mac or Linux hosts.