Power BI is not included in O365 E3, but we could use the Power BI free version without any charge. And if you would like to use Power BI Pro, then you should pay $9.99 / £6.20 per month.
Check the article below to see what is a Power BI pro content:
Power BI Pro content - what is it?
And there is no free or pro version regarding Power BI desktop, Power BI desktop is totally free.
Regarding how to buy Power BI pro, check:
Purchasing Power BI Pro
Also see the FAQs:
Frequently asked questions about Power BI
What smoupre suggest here is, as you are considering use both O365 E3 and Power BI Pro, it is better for you to switch to O365 E5 version, which includes Power BI, and this is more economical. See:
Office 365 Business plans
If any further assistance needed, please feel free to post back.