As far as vision is concerned, what is a prescription of -3.00 equal to in 20 terms?

Can't be done. Apples and oranges.

The 20 in 20/20 vision is 6 metres.
-3.00 is a diopter reading. In this case it is myopia.
Myopia is a visual defect in which distant objects appear blurred because their images are focused in front of the retina rather than on {it;} nearsightedness. Also called short sight

A diopter is a unit of measurement that indicates the amount of correction needed to change your vision to as close to 20/20 as possible. The numbers on your eyeglass prescription are diopters, and they indicate the type and power of lens needed to achieve normal vision . A numeral preceded by + (for hyperopia) or - (for myopia). Nearly 85% of nearsighted individuals are in the range of -1.50 to -7.00 diopters, which is considered mild to moderate. Moderately short sighted = (-1.00 to %963.00 dioptres)



( Extract:...the majority of whom will achieve 6/6 %96 commonly known as 20/20 vision.)


NOTE: Diopters CANNOT be mapped into visual acuity in ANY meaningful way
no more than apples equate to oranges. 20/XX is a method that seeks to give a
quantification to the resolving power of the eye with respect to standardized
letter sizes. This method, adopted 125 years ago, is quite simple. 20/20 means
that the tested eye can discern a specific sized letter at 20 feet ( 6 meters) - this is considered the norm. 20/40 means that the tested eye can discern a
letter of a size at 20 feet that a 'normal' eye can discern at 40 feet,

Conversely, 20/15 denotes superior sight because it indicates the eye can recognize a test letter from 20 feet away that an average eye cannot recognize from farther than 15 feet .
Dividing the numerator by the denominator gives you an idea of the visual
efficiency of the eye. Thus a 20/40 eye has a 50% efficiency - sufficient to
drive in a

Being a little bit verbose anain...
Conversely, 20/15 denotes superior sight because it indicates the eye can recognize a test letter from 20 feet away that an average eye cannot recognize from farther than 15 feet .
Dividing the numerator by the denominator gives you an idea of the visual
efficiency of the eye. Thus a 20/40 eye has a 50% efficiency - sufficient to
drive in all states, day or nite.

http://www.allaboutvision.com/eye-exam/2020-vision.htm