What is diff in application of 3 & 4 roller plate rolling machine

[vasim]

what is diff in application of 3 & 4 roller plate rolling machine.
which type of rolling machine more usefule for 6 mm plate...
& please suggest the ways to reduce the flatness of end portion after rolling ...

 

[Baluncore]

http://www.thefabricator.com/article/bending/a-rundown-on-rolling-machines

The four-roll styles have an additional roller underneath the top roll, which constantly pinches the plate during rolling. Double-pinch rolls can prebend both plate ends without removal, as is required with single-pinch rolls.
Three-roll machines generally require prebending the leading end, running the sheet through the machine to prebend the trailing end, and then switching roll rotation direction to roll the cylinder body. Four-roll plate rolls have a slight advantage in cycle time because they permit prebending of the leading edge, rolling the cylinder body, and finishing off the trailing prebend, all while rolling in the same direction.

Also see;
http://www.thefabricator.com/article/bending/a-plate-rolling-primer