module position and product display

[alfmayster]

Good morning to all the community and best wishes for happy new year, I am italian but in my section i didn't found an answer and in the following dilemma, I have set my index so that the category menu is displayed on the left as you can see from the attachment, but I cannot understand how or put 3 product columns or set up a side banner to cover that hideous and unsightly side space in the third product column, does anyone have any advice or suggest a guide on how to overcome my obstacle?

PLEASE SOMEONE HELP ME

THANKS