Unless specified in your contract this is discretionary, but it is usually good form to consult your manager as to whether and how they would want this news conveyed. What you want to avoid is the proverbial elation at moving to a new role and in any way diminishing where you are - not only is it demotivating to your colleagues, but it also burns bridges. It's only 3 months - assuming you have a positive relationship with your manager, and dont actually hate the firm you are leaving, have a conversation with your manager and ask them how they would like you to handle this. A good manager will appreciate the consideration and it marks you out as a good leaver.
