I've only got 2 powerline adaptors running in my house, but I'm 99% certain that would work fine. The adaptors broadcast across the entire power network in your house, so every adaptor will be able to 'see' the other adaptors.
My setup is: Router > powerline adapter 1 > powerline adapter 2 > network switch > ethernet devices.
Works great
It may depend on the brand you buy, but my adapters came with some software that lets you assign an encryption key to each device. Only devices with matching keys can talk to each other.
This in effect allows you to create VLANs, so you can segregate different devices into seperate logical networks if you need to (I very much doubt you will).
It obviously means you can also set an encryption key for your network to stop other powerline users getting in, just like you would with a wireless network.
This is only really necessary if you live in a block of flats, or any buildings that share the same ring main. If you live in a detached house, it's very unlikely that your neighbours would be able to connect to your network (I tried to get my elderly neighbours onto my network to share our broadband, but couldn't get a signal).
All the units are capable of transmitting and receiving, so there is no specific 'transmitter' unit.