Let follow the simple steps..
I believe, you have created some portfolio posts already..
– Go to Pages > Add New
– Enter the Title you want e.g. “Portfolio Three Columns”
– Select “Portfolio Standard 3 Column Template” from the “Page Attributes” in the right column
– Publish the Page
You are done..
Please create Custom Menu instead of using default one. Now you can public this page by assigning it to the Custom Menus.