Question

How tall is a triangle that is 10ft x 10 ft x 10ft?

Answer 1

Are you saying that all 3 sides of the triangle are equal, and are 10-ft each ?

In that case, if you set the triangle down on one of its sides,
the vertex at the top will be 5√3 = about 8.66-ft above the table.

Answer 2

Square root of 75 or 8.66 rounded. Use Pythagorean Theorem.