Insert a new full-width buffer underneath two vertically split buffers

If you opened a third window just focus the window you want at the bottom and press Ctrl+w J this will move the window under the other two.

Ctrl+w K - move window up

Ctrl+w L - move window right

Ctrl+w H - move window left

Or, to do it in a single command:

:bot sp

The long version of this is

:botright split

But I find that more confusing than the short version! I'll let you look at :help :botright for an explanation :-)