If you are the only girl hanging out with two of her friends, you may feel a little uncomfortable or awkward. However, if this boy invites you to hang out, he may have a crush on you or just want to be friends with you.
When you go out to play, you can try to relax, chat with him and his friends, and enjoy this time. If you feel a little embarrassed or uncomfortable, you can try to find some common topics, such as hobbies, music, movies, etc., to initiate some relaxed conversations.
At the end, you can thank him for the invitation and express your feelings. If you found it a pleasant experience, you can tell him that you were happy to hang out and hope there will be more opportunities in the future. If you feel uncomfortable or uncomfortable, you can also tell him so that he can understand how you feel and make adjustments.