White vertical lines are appearing at even spacing across the width of the screen. Any idea how to fix ?

This is screen problem .
You may be loosing somewhere on the screen of your screen, if not
If there is no losing then you change 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.

If you wish to get some details; check the site linked here. Pull up older posts. Surf the site with patience. http://electronicshelponline.blogspot.com/
Line, faults & possible causes to LCD screen are given with illustrations. You can get an idea about the fault to an extent.
Search the site by type in the "brand name" to your device, in the "Search box at the top right of the display window of the Homepage" to get gathered related posts.

I looked up how to fix red horizontal lines on a Philips flat-screen tv. It turns out it needed the Y buffer upper replaced. I am not sure if this problem would be similar but it might help. I am also searching how to fix a fuzzy screen on a DLP tv. Hope you find what you need.

check video memory and connectors.

that is due to lcd filter units , try to align, precision work

You have a defective light engine. Contact RCA support. Sometimes they will warranty these even after the 1yr warranty has expired.

if these lines are just at the top of the picture then i would suspect an electrolytic capacitor in the vertical circuit.

This is probably caused by the small signal board. It controls and decodes the picture information before it is sent to the display panel. It can be fixed by a quailified technician and if you contact philips (this is who makes magnavox) they will tell you the closest certified technician. Hope this helps.

Sometimes if the resolution is set too high, white lines will appear.