You need to be honest with yourself. Are you sure you want 'X'? or are you going to do the same with each school? You will be in danger because schools may think you are playing with them just for money and may get offended.
From its response, school x accept negotiations. From my point, you can mention the z school only or put a range for both schools Y and Z. try to estimate Y. My reason os that X & Z are same so Y may be around the same. I say Maybe. However, you need to be smart and back up your reasoning with examples and show them why school X fits you. Mention details, communication with students, clubs, alumni. You can provide them your financial situation. Do not shoot numbers or threat them because the response may come take it or leave it. If they offer direct contact, it would be nice. If not, you have to show through your mails.
I'm not supporting faking numbers. I do not know if they will contact the other schools. But they may be smart enough to weigh the amount with school offering that amount. Most probably school X will appreciate your honesty and offer more but show them strong reasons as said above.
I hope it helps.
good luck