The Pi3b+ can boot of USB without any SD card. Can also do this on previous model https://www.raspberrypi.org/documentation/hardware/raspberry... but as you can read, that would make a permanent change to enable via a one time programmable bit and remove the ability to boot from SD card.

Though as you have the 3b+, then you have none of that dilemma and it will just boot, done it myself.