That math is correct. Exposure is logarithmic, and multiplier is linear, so an exposure of three is 2 to the power of 3, which is 8:
2 * 2 * 2 = 8
To be more clear, if your original value is 1, then:
+0 exposure = a value of 1
+1 exposure = a value of 2
+2 exposure = a value of 4
+3 exposure = a value of 8
+4 exposure = a value of 16
etc.
It also works the other way too:
-1 exposure = 0.5
-2 exposure = 0.25
-3 exposure = 0.125
etc.
Basically when you are talking in exposures, in terms of whole numbers, you are doubling or halving the amount of light.