Although you were converting to liters which is fine, the numbers on a Benz specifically refer to the amount of volume displacement of the cylinders in your engine in cubic centimeters, i.e., my 420 SEL has 420 cm3, your 300 SD has 300 cm3. The American system is in cubic inches, i.e., my Olds has a 455 cubic inch engine, which is around 7.5 L, which would be 750 cm3. If my Olds were a Benz, it would be a 750 SL. From cm3 to liters, simply divide by 100 as detailed in the previous posts.
Here's a quick lesson on what the letters mean:
C (coupe or the smallest model)
T station wagon
SLC like SL, but hardtop - no convert
SEL largest body style gas engine
SDL largest body style - diesel engine
D Diesel mid or small body style
SD like SDL , but a little shorter in length
SE like SEL ,but a little shorter as well
The newer models have the letter come first, i.e., S500, don't know why. In any case, the best thing to do is to go to a dealership and look at the brochures. I am sure I left out a bunch, but the basic are there.
'89 420 SEL
'90 300 SEL
'68 Olds 88 Convertible
'84 300 SD (sold it)