I have a theory for both your questions:
Alien 3: Theres two options:
1: The egg spawned 2 facehuggers, like they can in aliens exctinction game, one found ripley and the other found the dog.
2: My fave theory: the egg released a queen carrying facehugger/assembly cut shows the super facehugger, and it impregnated ripley with the queen and after that it then impregnated a dog/ox with a protector to the queen that will burst beofre the queen does and will be aggressive and protect the queen.
For why the different speeds:
I count out avp movies: Covenant we have Davids strand of xenos, not the original xenomorphs, they arent biomechanical, their eggs looks different, born different, looks different etc and behaves differently and way more aggressive and dumber. But their facehuggers dont need much time to impregnate a host and Davids xeno burst quicker and is born like a miniatour of the adult shape and is transparent, can see the skeleton and organs in it.
In alien and aliens and alien 3 the facehugger is attached a long time and also the chestburster take longer to get mature and burst out, same for resurrection. How much they grow inside the host is individual and what kind of host it has and health of the host. The ones in resurrection and alien took shortest time and hence only looking like snakes while the dog runner in alien3 matured for longer time and had imature arms and legs.