What was the "Karkov"?
Rashi: There were actually two Karkovs; one was an ornamental groove one Amah wide that surrounded the Mizbe'ach immediately above the Michbar, the other, a sunken path beside the four K'ranos, one Amah wide surrounding the Mizbe'ach, to prevent the Kohanim from slipping.
Three Amos from the top - according to the opinion that the Mizbe'ach was ten Amos tall (Refer to 27:1:1:2). See Rashi, who elaborates on the implications of the word 'Karkov'.
How did the Kohanim arrive at the top of the Mizbe'ach to perform the Avodos that needed to be performed there.
Rashi: In order to climb on to the Mizbe'ach, they manufactured a ramp.
as the Pasuk hints at the end of Yisro.
That reached as far as one Amah from the southern wall, according to R. Yossi, and was only ten Amos long according to R. Yehudah (Refer to 27:1:1:1 and refer to 27:1:1:2. The top of the ramp stopped a hairs-breadth away from the Mizbe'ach (See Zevachim, 62b [Rashi]).