A:AnswerHeadphone jack, No. Does have optical out for audio. Not sure what you mean by three headed cable, but it supports RGB + L/R audio ( red, green, blue and red/white audio) or composite video + L/R audio (yellow video and red/white audio). Yes, it has an F type connector for either cable (for those places that still let you do that), or an external OTA antenna. Using the antenna myself, and am pleased with the receiver.
A:Answer36.1". Info in specifications, It is recommended in the owner's manual to have a specified amount of additional inches of space on each side for ventilation to avoid a fire hazard. especially in a cabinet. Mine's on top of a tv stand .
.