I have just converted my FT Edge 540 from a twin to a tri prop.
I won't go into the math's but I was originally running a 12x6 twin producing 1450g of thrust with a specific motor & battery setup. Unfortunately following some previous crashes and modifications I've ended up with vibration issues when I throttled up.
As I didn't have a spare 12x6 to swap over and couldn't be arsed balancing the prop and spinner, I opted for the next best thing that I did have an 11x5 twin prop. The calculated thrust for that prop would have been 1350g, so no significant drop.
However, to run an equivalent 3 blade prop and get roughly the same 1350g of thrust I calculated that I would need a 10x5 to achieve this. As I already had a 10x5 three blade prop, I fitted it today. Low and behold the vibration has gone and the plane feels like she has her thrust back win, win.
Tri blades have there uses and may not be as inefficient as you thought