Hi,
As an Office 365 E3 user, you will need a license in order to create or use apps with premium connectors. However, as a Power Apps developer you could leverage the Power Apps developer plan or the Power Apps Trial license in order to try out the development with premium connectors.
As an IT admin you can check the currently active licenses via Azure AD. The user can also see the active licenses within the Power Platform, by selecting the gear icon on the upper right corner and clicking on 'Plans'. It would also be worth checking whether a Pay-As-You-Go subscription has been set up in that environment - that could also explain the access to premium features.
The developer can start trying out the features with these plans but will need a premium license down the road, albeit Per User, Per App or Pay-as-you-go (environment level).
If this solves your question, would you be so kind as to accept it as a solution.
Thanks!