Attacker: one die; Defender: one die:

Attacker wins 15 out of 36 (41.67 %)
Defender wins 21 out of 36 (58.33 %)


Attacker: two dice; Defender: one die:

Attacker wins 125 out of 216 (57.87 %)
Defender wins 91 out of 216 (42.13 %)


Attacker: three dice; Defender: one die:

Attacker wins 855 out of 1296 (65.97 %)
Defender wins 441 out of 1296 (34.03 %)


Attacker: one die; Defender: two dice:

Attacker wins 55 out of 216 (25.46 %)
Defender wins 161 out of 216 (74.54 %)


Attacker: two dice; Defender: two dice:

Attacker wins both: 295 out of 1296 (22.76 %)
Defender wins both: 581 out of 1296 (44.83 %)
Both win one: 420 out of 1296 (32.41 %)


Attacker: three dice; Defender: two dice:

Attacker wins both: 2890 out of 7776 (37.17 %)
Defender wins both: 2275 out of 7776 (29.26 %)
Both win one: 2611 out of 7776 (33.58 %)

Attacker 1 versus defender 1: defender has the advantage, winning about 4 out of 7 battles
Attacker 2 versus defender 1: attacker has the advantage, winning about 4 out of 7 battles
Attacker 3 versus defender 1: attacker has the advantage, winning about 2 out of 3 battles
Attacker 1 versus defender 2: defender has the advantage, winning about 3 out of 4 battles
Attacker 2 versus defender 2: defender has the advantage, winning about 3 out of 5 battles
Attacker 3 versus defender 2: attacker has the advantage, but the advantage is much more narrow than any of the battles described above.

http://datagenetics.com/blog/november22011/

All I can tell you is what happened to me dude.