Two horizontal lines going across the screen

To clarify, are the 3 black lines horizontal (left to right across entire width of screen)? or are the 3 black lines vertical (top to bottom across entire height of screen)?

Horizontal lines are almost always the result of a failed panel, and no board replacement will fix the issue.

Vertical lines such as you have described (only 3) are probably also a bad panel. If the black lines were evenly spaced in a repeating pattern, it could be a mainboard or TCONN board.

Has this panel been cracked? or perhaps cleaned recently with too much water or cleaning solvent that has gotten the screen wet?

You may have to sell the TV for parts.

A defective display panel.

are the lines going left to right or up or down? if up or down probaly a bad panel,iff left or right either the upper or lower buffer board is bad

ON THE EXTREME LEFT SIDE OF YOUR TV FACING BACK OF TV WITH COVER OFF THERE ARE TWO BUFFER BOARDS UPPER AND LOWER WITH WITH MANY FLEX CONNECTERS CLEAN THESE CONNECTERS WITH CONTACT CLEANER UNPLUG FLEX CONNECTER BEFORE CLEANING CONNECTER ONLY DISCONNECT CONNECTERS THAT ARE ADJESCENT TO HORIZONTALE LINES THESE CONNECTERS ARE NOT EASY TO DISCONNECT AND RECONNECT IF THIS DOES NOT SOLVE PROBLEM PANEL IS DEFECTIVE

The line is there because the vertical deflection unit - the one that spreads that line across the entire screen to form the image - has failed. Replacing it is a job for a professional, or at least an experienced amateur, and the parts will be quite difficult to find for such an old tv . 

panel t-con parts n/a will most likley be replaced.

Loose connections in the AV board. Re solder . Did you try in video 2 or 3 ?....But Am suggested re solder AV board and connectors. I hope this help you I am here any time thank you!!!

When I turn the TV on, I get a horizontal line across the screen approx 1/3 from the top. My TV make is Diamond and is approx 6 yrs old

Sounds like the flyback transformer is bad. couple of 100 dollars if it is that

I had this problem around the time my TV was 3 years old. I ended up having to get it repaired at a TV repair place, it cost me $375.