How to hide the right and left regions from a customized login page?
Go to the Blocks admin page, admin/structure/block.

Find the regions you don't want to show. E.G., 'Sidebar A,' 'Sidebar B.'

'Configure' the blocks which appear in those regions. They will all have a 'Pages' tab at the bottom, and this is where you can configure which pages contain or exclude those blocks. You'd check 'All pages except those listed' and then put the path to your custom page.

HTH.