The Microsoft 365 Enterprise plans are a convenient way of purchasing Windows 10 Enterprise, Office 365 and EMS. If you look at the difference between the E3 and E5 plans then there are three main areas where you get more functionality in E5: security and compliance, productivity tools, and telephony.
On 2nd January, 2019 Microsoft announced two new Add-on plans for Microsoft 365 E3 to allow customers to buy the security and compliance components at a cost-effective price from 1st February, 2019.
Information Protection & Compliance will be made up of Office 365 Advanced Compliance and Azure Information Protection Plan 2 and will be $10 per user per month.
Identity & Threat Protection will be ALL of the other security components: Windows Defender Advanced Threat Protection, Azure Advanced Threat Protection for Users, Office 365 Advanced Threat Protection (now including Office 365 Threat Intelligence), Azure Active Directory Premium Plan 2, and Microsoft Cloud App Security. This plan will be available for $12 per user per month.
Find the Microsoft announcement here: http://bit.ly/2sFNgUa.