I did a group tour a couple years ago. One guy had a low slung trike with a trailer. I was really surprised how poor the visibility was for the trike with trailer because he was so low to the ground. He used one flag that really improved his visibility. I agree with the suggestion above, a flashing light on the left side should be enough. But, make sure that the flashing light has good batteries, a dim light with low batteries can be a hazard because it makes you think you are safe when it is not very visible. I do not think three flags are necessary, but one or two are really good ideas.

If you are riding a normal bike where you are sitting on the bike up higher, that is much more visible, especially if you are wearing high vis clothing. In that case, you and the trailer may be more visible than most bikers by themselves. On that group tour, two had Bike Friday bikes with trailers, they were much more visible than the guy on the trike with trailer because with the Bike Fridays, they were up high where they could be seen.

The three of the bikers on that group tour that had trailers (one trike, two Bike Fridays), they all were a bit further into traffic than those of us that had panniers because all three of the trailers were of the two wheel variety and both wheels had to be on solid pavement. But I think all three of those trailers were almost a foot narrower than your trailer. Do you really need that wide of a trailer?

There have been a few times where a semi is trying to give me plenty of room, but suddenly on coming traffic from around a bend appears and the semi pulls back and really crowds me. And a couple times I have ridden off the pavement without incident. My point is that even when traffic is trying to give you plenty of room, the situation can change. And that is when a wide trailer might be problematic.

I wish you luck in your endeavors.