For Office 365 Attack Simulation Training, it's required to have either Microsoft 365 E5 or Microsoft Defender for Office 365 Plan 2, but these licenses are required for the users you are testing, not just the admin. Therefore, if you are planning to test all users, all users will need to have either one of these licenses.
However,
Attack simulation training offers a subset of capabilities to E3 customers as a trial. The trial offering contains the ability to use a Credential Harvest payload and the ability to select 'ISA Phishing' or 'Mass Market Phishing' training experiences. No other capabilities are part of the E3 trial offering.