Microsoft is allows you to add Microsoft Bookings to E3 plans, but its not enabled by default.

To enable it, log into you Microsoft Office 365 Admin Center

Click Billing>Purchase Services

Under the Add-on Subscriptions Section, select the Business Apps (Free) Product
Choose the number of licences you require and submit.

Once that’s complete you will then need to assign the licences to existing users.

So Click Users>Active Users

Select the user you want to activate it on, and click edit beside the Product Licenses.
Turn on Business Apps (Free) and hit save.

Wait a minute or two and Bookings will now show up in your app draw.