You have to make three attack rolls to get the best damage. Possible, but not nearly certain. Why would you ever choose that over the other powers that hit for that much with one attack roll? Okay, if you wanted to improve your probability to hit, I suppose it might be worth it.
On further thought, I could possibly see how it would work if you wanted to absolutely hit your shroud target to deal the damage from that. It works even better if you had another rider like Sneak Attack to maximize the damage. Even then, Flurry of Talons is better, since it targets Reflex, is non-typed damage, and the non-OA flight is better IMO than invisibility. I just can't see how it would be worth it.
Mathematically, Shadow Fire is better than a regular 3[W]+Dex attack. It would be even if each attack contributed 1/3 of the total damage, but that's not the case. Each attack contributes 1[W], but the static modifiers all kick in with at least one hit. This weights the expected damage heavily in favor of Shadow Fire.
I can break it down, I guess. If you want the numbers, just read on.
Lets say you attack with a 1d8 weapon, and you have +10 from static modifiers.
If you use a 3[W]+dex attack, your average damage on a hit will be 23.5. So your expected damage is p*23.5, where p is the probability of a hit. If you hit 70% of the time, that's 16.45. If you hit 50% of the time, that's 11.75.
If you use Shadow Fire, your expected damage with one hit is 14.5. With two its 19. With three its 23.5. But your chance of hitting has to be divided out by category. If you hit 70% of the time, your chance of getting a certain number of hits is,
0 hits: 2.7%
1 hit: 18.9%
2 hits: 44.1%
3 hits: 34.3%
.027*0 + .189*14.5 + .441*19 + .343*23.5 = 19.18
That's a win by about 3 points of damage for Shadow Fire.
If you hit 50% of the time, things change a bit
0 hits: 12.5%
1 hit: 37.5%
2 hits: 37.5%
3 hits: 12.5%
.125*0 + .375*14.5 + .375*19 + .125*23.5 = 15.5
Now its a win by 4 points. Generally speaking, Shadow Fire starts ahead, and then pulls further ahead the worse your chances of hitting become.