My advice is stick with 2.5mm ID. 3mm will fit but the deck isn't really designed lor large coils. The deck is small and the airflow vents are small. It's a Nano, after all. Plus, you don't want thick cotton tails. Very easy with this rta to use too much cotton in the wick ports.
Ive been playing around with mine since I got it trying various coils at both 2.5 and 3mm. The best results I've had by far is with Ni80 25ga flat wire. 2.5mm ID, 6 wraps, 0.5 ohm @40w. Airflow closed off 50%.
It's a little banger.