USCG Rules say (for both inland and international):
Section 23(d) (i) A power-driven vessel of less than 12 meters in length may in lieu of the lights prescribed in Rule 23(a) exhibit an all-round white light and sidelights.
The international rules also add:
(ii) a power-driven vessel of less than 7 meters in length whose maximum speed does not exceed 7 knots may in lieu of the lights prescribed in Rule 23(a) exhibit an all-round white light and shall, if practicable, also exhibit sidelights.
The rules don't say anything about the need to extinguish a sternlight if you have one.
My little 19' Menger catboat's stock nav lights do not include a steaming light -- just sidelights, sternlight and anchor light. The sidelights and sternlight are on the same switch, and the anchor light is on a separate switch.
It looks to me like the obvious intent is to use just the sidelights and sternlight when sailing, and to add the "anchor" light as a steaming light when under power... which to my reading meets the letter and intent of the rules above.
Except you're showing two white lights aft. Only one is allowed. That's why I said of you're using your anchor light as a steaming light the stern light should be extinguished.