Total sessions or active users for multiple websites or GA4 sources

Hi all,

I am new to Looker studio. I am trying to make a marketing report to show the new CEO and Director of marketing the success of the Google ads, SEO and other digital marketing efforts. They have a very limited knowledge of google ads, and get overwhelmed by the data I have been trying to show them. They want me to make a more simple report that can show that the efforts are worth it.

I want to make an overview page that has the total website traffic for our 3 main websites, the total leads for all websites, and other totals. I cant figure out how to add the totals from multiple data sources. Maybe i dont understand blending data sources or how to use a formula. 

Could someone explain out how to do this?