You are talking about pixels but you also use the keyword Adobe Illustrator so this is confusing. You can use mathematical expressions in many parts of Adobe softwares so if you want perfect thirds, just add /3 to divide in thirds.
I'm not sure what you want to do with this afterwards so I'll give you a way to do it that looks like what you've started :
Use a square and FOUR lines instead of 2.
Put the top and bottom lines exactly on your square.
Select all lines and use the align panel : distribute vertically on selection.
Erase top and bottom line.
Use the pathfinder to remove the leftover lines outside your shape.
Using smart guides would be helpful (cmd+u)