Yes, this pulsing is normal. Once fully charged, it turns green. The LG G2 also does this.
I have it disabled and am using Light Flow to indicate charge/charged. One of my few disappointments with the G3 is the small LED size and narrow viewing angle of the LED. I count on the LED to tell me if notifications are pending but it's virtually impossible on the G3 since the LED is so small, not overly bright and you must almost be looking at the device head-on. Woops, sorry for the derail...