You can
buy (for 3 camels) only 1 black dice per turn.
There is no limit for black dice you can get in your turn in other ways (completing a contract or gaining city bonus).
Thus you can get even 4 black dice in your turn (buy for camels, complete two contracts and enter the city with this one-time bonus) - if there is enough black dice in the pool.
Remember that you are restricted by the number of availible black dices. If you complete a contract (giving black dice) when there is no black dice availible, then you don't gain it, neither now nor the next time black dice are availible. Thus better to wait with completing such contract.