2 double flushing toilets and 1 slow draining toilet - happened out of nowhere

Hey guys - I'll try to be brief.

We've been living in this house for just over 1 year and have had no problems with the plumbing. About a month ago, one day we noticed that two of our toilets were flushing twice, out of nowhere. We'd flush the toilet, the flapper would stay up, the bowl would drain twice (with a slight pause in between each flush) then the flapper would close and the bowl would fill.

At the same time this started happening, our master bathroom toilet started flushing/draining extremely slow. During the flush, the water would fill all the way to the top of the bowl and then slowly drain.

I used some commercial strength drain-o and a lot of plunging and the toilet started flushing normally again although the other 2 toilets were still flushing twice. A couple of days later however, the master bathroom's toilet started doing the same thing again. We have repeated this process a couple of times now and the steps are the same. Meanwhile, the other two toilets are still flushing twice.

To be clear about our setup in our house, 2 full bathrooms upstairs on opposite sides of the house (1 with toilet that is slow flushing and 1 with double flushing toilet). There is a half bath downstairs that has a double flushing toilet. No bathroom is directly above the downstairs half bath. We have NO draining issues on any other fixtures in the bathrooms. Sinks and bathtubs drain completely fine.

I'm wondering what would have caused this to just miraculously start happening? My mother has asked around to a few plumber friends and they are suggesting that the flapper is the wrong flapper. My question would be why wouldn't this have been a problem the entire time and not just occurring randomly one day?

我将尝试通过脉冲运行一条鱼es from my room and at the toilet levels, but if anyone has any other input or advice, it would be greatly appreciated!!