all toilets suddenly drain slowly

[raevsky]

I have a strange problem. Several days ago all my toilet bowls in my house (three toilets) started draining slowly versus regularly being rather fast. That was simultaneousely for all three of them.
That is the case with toilets bowls only, not with the sinks or baths.
For toilets it is exactly the same with clean water and with anything else.
The strange thing is that if I open all possible faucets in my house at the same time, the toilet bowls drain fast.
I tried to treat my plumbing system with enzymatic drain cleaner - for two days (I am going to do it one more day), and no improvement yet.
I have a suspicion there is air in the plumbing system.
Any ideas?

[raevsky]

Out of the three toilets one is located on the first floor, and two others on the second floor almost directly above the first one. Also, I frequently see air coming out of the toilet bowls when flushing.