Why not go for something like a Raven 90cm Gemini dish with an offset bracket to receive Astra 28.2, 19.2 and Hotbird 13E, a DiseqC 4 way switch and you have a nice selection of channels. Othersie a compact DiseqC motor solution with a 90cm dish, gets you even more.
With a fixed dish, you need a separate LNB for each satellite - the three signals hit the dish from different angles, bounce off in different directions and are captured by the LNB's, carefully positioned along the offset arm.
Each LNB will need an output for each receiver. You also need a diseqc switch for each receiver - to select which LNB is connected to the receiver at any given time.
Here's an example: Suppose you have two decoders and you want to receive three satellites. You need 3, twin-output LNBs and 2 (3 way or more) diseqc switches. You connect output "A" of each LNB to switch "A" and connect that to receiver "A". Then you connect output "B" of each LNB to switch "B" and connect that to receiver "B".
If you have four receivers, you would need 3, quad output LNBs and 4 diseqc switches.
To capture the maximum signal, your dish needs to be directly facing the satellite, obviously this is not possible if you are trying to receive 3 satellites on a fixed dish.
Better solutions would be either a separate fixed dish for each satellite, or a separate motorised dish for each receiver. But planning bylaws typically limit you to one dish, less than 1m diameter.
Sorry lowdown, read your posts again and see that you intend 4 receivers, wasn't 100% clear from your original post. As Simba27 says, you will need as many DiseqC switches as receivers and either Twin, Quad or Octo LNBs on each satellite. From SW London, a 70cm Raven Gemini or Viasat Bisat with additional sat bracket setup would do the trick.