I'm just 12 years old but can anyone answer this question?
What is the "three dimensional diagonal" of a rectangular prism with the dimensions
3^4^12 (this is actually 7th grade math).:)

I assume this is 3 x 4 x 12. That makes it a rectangular solid (rectangular prism). I tried to draw one, but I'm art-challenged so here's a link to look at:

Rectangular Prism (https://math8geometry.wikispaces.com/Drawing+a+Net+for+Rectangular+Prisms+and+Square+Py ramids)

The dimension you want is a line from the rear-left-bottom to the front-right-top. If you also draw a line along the base from the bottom-left-rear to the bottom-front-right you'll form a triangle using the right front edge of the prism. It's the diagonal of the triangle that you want to measure.

Since the base is 3 x 4, the diagonal of the base is 5 (3-4-5 right triangle, or you can figure it out using the Pythagorean theorem).

\sqrt {3^2 + 4^2} = 5

This is one leg of the resultant triangle. Since the height of the prism is 12, this forms a 5-12-X triangle. If you aren't familiar with the 5-12-13 right triangle, you can figure out the length, "X", from the Pythagorean theorem. The length of the "three dimensional diagonal" is given by

\sqrt {5^2 + 12^2} = 13

Very Good