When clicking on an account, the client has an activity tab on the right side of the page. The activity currently has 2 tabs under it. One is for New Task and another for Email. The client wants to add a third tab under the account activity. What is the right way to create tab under account activity? Please note this is not opportunity activity. It is account activity.
That part of the layout is governed by quick actions (which are a subset of global actions) and you can customise them based on 'Salesforce Mobile and lightning Experience Actions' in the Page Layout for the Account Object.
NB: Before adding the Quick Action, you'll need to set it up under 'Global Actions' in the main Setup menu.