Please give me likely outcome when I ask for 3 mos off for hip surgery (details in text)

I was off for 4 weeks in April for hernia surgery. We are spread thin of drivers, and it caused hardship for the other drivers because they had to work extra time to cover for me. (I am a truck driver, and we only have 4 full time drivers, plus the driver manger. We are hourly, and yes we get OT. No we are not union).

Recently, they have been asking me to work 6 days a week so other drivers can go on vacation or take personal days. I tell them I don't want to work 6 days a week. I just want to work 5 days. They are not pleased with me, even though I do a good job as far as my duties are concerned. They can count on 5 days from me. I never call in sick. I just refuse to work 6 days. So why would they agree to give me 3 months off for hip surgery at the end of this year when I won't work 6 days to cover for another driver?

I have been employed with them for 13 months so I do qualify for FAML (or whatever it's called). It is a large company. Please give me a couple of scenarios about how this would go if I were to ask for time off for hip surgery.

"Hey boss I have severe arthritis in my hip (the managers are fully aware of this since I walk with a limp), the surgeon said he can do the surgery in December. Is it ok to take off 3 months for recovery"?

What's the likely scenarios for this? Can they say, no? If I plan for the surgery regardless, can they find other reasons to fire me before the surgery so that my health insurance becomes null and void. Please help me understand.