First, it's your parent's home. They get to set the rules, including if you have pets, how many, and what kind. Second, you already have two cats, don't be greedy. Every single pet needs time, attention, energy, and money, as part of their care. Two pets need twice as much of all of this, as one pet. Three pets need three times as much of everything, as one pet. And don't kid yourself that you will do all of everything related to this kitten all of the time. Your mother would probably be the one doing most of the work related to the new pet, just as she is probably doing for the existing two cats. You should be thankful that they already let you have two cats. Don't add to the household expenses, and the household work load. And, don't give your parents a hard time on this. LET IT GO! Once you are on your own you can have more pets. Just make sure they are fixed.
Remember, we on the spectrum are all:
A Different Drummer
If a man does not keep pace with his companions,
Perhaps it is because he hears a different drummer.
Let him step to the music which he hears,
However measured or far away.
--Henry David Thoreau