Within episode 3 of 'Humans', there is some use of semiotics. These are used as a way for the encoders to give certain messages, that the audience subsequently decode. Semiotics can also be used to give shortcuts to meaning, with an example of this being the often green eyes of the synth characters and the deeply pale skin. Green often signifies jealousy and in episode 3 specifically, we see the synths becoming increasingly concerned with humanity and how they long for even a shred of it. Again, this is an effective use of a shortcut to meaning but also gives the show layered messaging, rather than just overtly telling the audience how the characters are feeling. Symbolic codes are used here to specifically touch upon the themes of identity, what it means to be human and also the binary opposition of man vs machine. The synths look like us yet we don't accept them as human and even when we see they can display emotions, we still don't quite accept their humanity. It's interesting that the show chooses to use the eyes to give us this extra details about the experience of the synths because eyes are often viewed as 'windows to the soul'. This almost implies the synths have souls, yet they are not flesh and bone so we cannot see them as our own. It makes us ponder the difference between an algorithm and human biology.
There is also prominent use of referential codes. The most overt f these is the mention of Asamov and his laws of robotics, which have clearly been implemented in this parallel world (based on character dialogue). Asamov is a well known sci-fi writer and part of the essential canon for many people who are interested in robots and synths. Mentioning his name helps pin the genre of this show down somewhat and ensures that we don't get lost in the haze of family drama.