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 :-)