Congrats on getting your double pirouettes solid on both sides! Kudos to you! What I normally tell my students when they’re working towards a triple pirouette is this: Give yourself a little bit more momentum as you begin your turn, use your plie, and engage your abdominal muscles even more after your double turn, in order to maintain your position for a 3rd turn. Also, use your breath. An extra inhale, pulling up, and engagement of your abs, while maintaining your position, will make getting around for that 3rd turn a breeze!
Hope this helps! :)