You need to specify your AFL output directory with --fuzzer-directory, and your peers with --peers. Some other options exist to let you fine-tune your afl-transmit experience, have a look at them via --help.

On default, afl-transmit opens port 1337/TCP to wait for incoming connections. If you are not on a private net, make sure to protect this port with a firewall, or anyone on the internet may send you files (although this might become interesting). As a countermeasure, use the --restrict-to-peers flags to only allow connections from your known peers.

Quickstart

On your host 10.0.0.1: ./afl-transmit --fuzzer-directory /ram/output --peers 10.0.0.2,10.0.0.3
On your host 10.0.0.2: ./afl-transmit --fuzzer-directory /ram/output --peers 10.0.0.1,10.0.0.3
On your host 10.0.0.3: ./afl-transmit --fuzzer-directory /ram/output --peers 10.0.0.1,10.0.0.2