No way to be sure-- just make sure you have enough time to supervise the new bird in the event that it needs separate out of cage time from your budgies (each day for a few hours) due to conflicts--- it could also lose a toe if it lands on your other bird's cage and they are bonded..so be extra watchful for aggression etc, and never totally trust them together due to the fact that you have a pair + new larger bird.
There could be jealousy, hormones, etc...jealousy in terms of you, or the other bird pair..Hard to say. They could also be fine, but if you get a baby, remember that when they mature sexually, behavior can change a lot, so don't just trust your birds together indefinitely because they get along when he is a baby.
Make sure you quarantine your new bird in a far away room for 45 days and wash very very well between touching birds, water, food, toys etc (consider changing shirts between handling), as asymptomatic diseases often shed during times of stress (adoption etc). You will want to get your current birds checked for diseases (blood work etc if possible) before you bring the new bird home, and then take the new bird when you can and before they mingle. Cockatiels produce a lot of dander and that can spread certain diseases in the air.
It is important to know that very healthy birds can carry very bad viruses (and spread them) even if they never show symptoms. Some birds can get sick with this between 1 week to 10 years of exposure, and others live their entire lifetimes as carriers without showing any symptoms.
Amen to no un-weaned babies.
