3X what a Hiroshima device can do.
That's not quite true, in terms of damage caused. Larger yield explosions are to some degree, "less efficient" than smaller ones. A crude way to think of it is that the destructive effect of an air blast goes up by the square root of the explosive power. (I know someone will come up with a more accurate figure, so, again, I just give that as a crude calculation.)
If you have enough material to make two small bombs (and a way to deliver two, to two targets), that's usually much more effective than a single larger "2x" bomb.
I'd be curious as to what type of design was found. Almost everyone forgets that a "gun" type A-bomb is not that hard to make, and is almost fool-proof, except that they can go off a little TOO easily...