Cancer is a sign very much associated with home. House, apartment, land, garden, home town, homeland. I often find crab shells in the water, which are reminders that Cancer carries her/his 'home' as part of himself or herself.
The Third House, at its core, is about being heard and read. Thus we go into areas like literacy, translation, speech impediments, deafness, public speaking, and of course vision, as one has to read, in order to translate and transmit.