3/4 means there are 3 quarter notes in a measure instead of 4 (as with the default 4 /4); it doesn't change anything about the length of those quarters. So the rows per beat don't need to be changed at all, just the rows per measure. You can calculate the rows per measure as (rows per beat) * (quarters per measure) / 4, so if there are 4 rows per beat, you get 12 rows per measure. Technically 6/8 is the same but typically interpreted as a more lively variant of 3/4, so doubling the number of rows per beat can make sense for 6/8. This means you'd get 8 rows per beat and 24 rows per measure.

Using 6 rows per beat isn't required for any of that, this is more often used if you want to use triplets in your music (dividing a quarter note into 3 24th notes instead of 4 16th notes). If you were going to use 6 rows per beat, you'd end up at 18 rows per measure as per the formula above.