Hi @Dereklad thanks for your post here in the Community.
As @newapollo has mentioned, a third outlet would be £5 per month extra but would be subject to the technician charges due to us sending those out for install, to check signal levels to the box itself.
I don't believe we've answered your question regarding the line of sight to the box though, and in this case yes, in order for the remote to work - yes this would be required for it to work.
Unfortunately, you can't tether the second mini box to the third so to speak, they would need their own individual connections via to the Virgin Media coax wall socket.
In terms of cable, it depends what kind of cable you mean but generally, it's quite standardised sizing but we can make lengthier ones if required but with your further questions, it would make sense for the technician to help get this installed for you, even though you do appear very confident in this field.
I'd advise calling the team on 150 / 0345 454 1111 and they'll be able to get this booked in for you if that's what you require.