First, open to the set up tab of the campaign you'd like to add additional product recommendations to. From there, Scroll down to the Product Recommendations section and choose your preferred product recommendation type.
To display more than 3 of this product recommendation type you will then:
1. Click the gear icon on the right so see the advanced configuration menu for product recommendations
2. Choose the number of product images you'd like to display in the "Number of recommendations to pre-generate (may require template changes)"
3. Navigate to the Design tab of the campaign. You will now see multiple options for product recommendation modules that will allow you to display the number you've selected in your settings and lower.