the first thing u need to do is to try to leave them outside every day with the cockateilsif they start fightingdont get worried every cockateil protects itself from a ather bird that it never newit might take 3 years2 years at least
You can try getting them a larger cage or having them spend more time together outside of the cage, also make sure everyone has their own food and water dishes. In all reality though, parakeets (I'm assuming you're talking about budgies?) are much more belligerent than cockatiels and it may never work. Keep in mind, birds go through different lifestages as well and if you have female budgies reaching sexual maturity, good luck.
It will also result in dead or badly injured chicks as the spare female will destroy anything in the nest in an effort to have the nestbox for herself. Its the most stressful way to breed budgies. But having one pair only in a breeding cage with their nestbox is fine. Budgies need to be older than 12 months and under 4 years. http://revivagenixfacts.com/