I wouldn't even say you are extremely unlucky... 6v3 is only a 76% chance to win the territory. That means 1 in 4 times you attempt it you will fail, which is really not all that uncommon at all.
This of course assumes you continue rolling until you have 1 army left... most players will stop before then meaning in practice the odds of taking a card on the first turn against a 3 are even lower.
In general it is wise not to roll less than 2 dice against 2 (ie 3 armies on your territory, 2 on your opponents) unless you are really desperate, as you are only 36% to conquer a territory using this method.
Generally you are better off only attacking 1s for a card if possible. 4v1 gives you a 90% chance of succeeding and 5v1 97%, and it goes up from there. Still if you play enough you will get plenty of times when you lose a 5v1 or worse - since 3 out of every 100 occassions is not that low a failure rate really.
