AC3 (Dolby Digital) - "AC3" alone does not tell us much because this can range from a 1.0 single-channel setup up to 5.1 surround sound. AC3 is just the codec package name for Dolby Digital so use "DD" followed by the channel setup. If there is a "LFE" (Low Frequency Effect, AKA the subwoofer) in the list of channel positions, then there is a ".1" and then count the other channels to obtain the front part of the channel count.
There is also "Dolby Digital Plus" which is nearly the same except it uses "E-AC3" codec.
DD2.0 (no LFE, 2 standard channels, 2+0 = 2 channels)
Format : AC-3
Format/Info : Audio Coding 3
...
Channel(s) : 2 channels
Channel positions : Front: L R
DD5.1 (1 LFE, 5 standard channels, 5+1 = 6 channels)
Format : AC-3
Format/Info : Audio Coding 3
...
Channel(s) : 6 channels
Channel positions : Front: L C R, Side: L R, LFE
DDP2.0 or DD+2.0
Format : E-AC-3
Format/Info : Enhanced Audio Coding 3
Codec ID : A_EAC3
...
Channel(s) : 2 channels
Channel positions : Front: L R