I have 3 routers : A,B and C

You only hope is to run WDS and I still suspect you are going to have massive problems. You are running a repeater off a repeater. It can be done but the main problem is WDS is not a actual standard. The manufactures have gotten better about their stuff working together but how you actually make a repeater run off a repeater is still very blurry how it should be done.

Lets say you figure out the configuration issues you are still likely to have performance issues. You lose 50% of your speed with just a single repeater running 2 you would lose another 50% and I suspect much more than that. You are also going to make the issue of a device connect to A can not hear a device connected to C so the both transmit at the same instant destroying each other signals must worse than when you only have one repeater.

You can't use AP mode because that only in effect bridges wired connection to wireless and the wireless may only have a single mac connected to it.

You might get better results if you used a device that act as a router but uses a wireless WAN connection. This is different than a repeater because all devices are hidden behind a single mac address and they generally use 2 different radios. 1 to connect to the main router and a second to connect to the end users. Running multiple of these chained would not have the massive repeater penalty and since they appear as a standard client device to the upstream router you odds of making it work are better because it does not need any special software support. There are limited numbers of routers than can use the Wireless a WAN port mostly third party firmware and specialized outdoor radios. You could of course use a hardware client-bridge device on the wan port of router b to connect to A and another client-bridge device on the wan port of router c to connect back to router b. You would have issues sharing data between the end machines because of NAT and different subnets but the performance to the internet would be better.

I would try any other form of networking..ie powerline or ethernet cables before you go down this path it will likely be a huge pain to get working.