The Hc12 is a bidirectional transceiver, but only at the data input output port.
The RF link is uni directional, and has to be periodically turned around to allow the data port to appear to be bi directional.
The type of data you are sending, the data rate and any pauses in the data being sent will impact on the ability of the data to be received properly.
Try reducing the data rate of whats being sent, and leave gaps in the data so its not continuous, and see if that improves.
The more receivers there are, the more the transmission link has to be reversed.
A HC12 is not really a good solution to what you are trying to do.
You would be better off with something simpler like one of the cheaper 433 Mhz OOK transmitters and receivers using Virtualwire.
The only other way to make this work is to set each receiver on a differant radio channel and then cycle the transmitter through the differant channels transmitting the same data each time.