Most shells don't have great support for making even non-linear graphs of (anonymous) pipe connections (though note that wilder, even dynamic graphs are possible with the "coproc" builtin of e.g. ksh, zsh, or modern bash).

However, arbitrary graphs are possible with the system calls available -- just create all the pipes in the spawning process with pipe(), and after fork(), but before exec() move the right ends into the file descriptors 0, 1, and 2 with dup2().