Just the users Manager - to all three it might be easier to illustrate in separate calls using variables. Once it's working you can always dump the variables and use a single-line function, but I find the variables helps to make sense of it three weeks later when you're looking at the code again 🙂
So your end result might look something like this (or you can convert it from chained commands to a single line), assuming you want to send to both the manager and manager's manager and cc the user. Obviously you can change this to any combination of To, Cc or Bcc;
UpdateContext({UserManager: Office365Users.Manager(User().Email).Mail});
UpdateContext({ManagerManager: Office365Users.Manager(UserManager).Mail});
Office365.SendEmail(UserManager & ";" & ManagerManager,"Subject",emailTemplate.HtmlText,{IsHtml: true, Cc:User().Email})Multiple values in To, Cc or Bcc are, like Outlook, separated by semi-colon ";"
Bear in mind, this relies on the Mail property of the Office365Users.Manager() function - if you have different Mail/UPN/Alias values for users it might not find the user- in this case you might want to return instead the managers id property for UserManager and use that to get both his Mail and his managers Mail.
Hope this helps,
RT