Ashen, I would say those are maybe day 15.  I would candle them every day for the next few days until you see the beak appear in the air cell....then lockdown super fast...look for twitching just under the air cell in the meantime, that's the only place you'll see movement now.
I had been going to say if they were 3 days different just to stop turning all at the day 18/15.  Turning is mostly critical from something like day 3 to 12, less of an issue later on.
Re.  Thermometers, if space is an issue, look for a probe one you can dangle in through an air vent.
Welcome CG!
I am candling tomorrow, but a lightening strike has taken out our Internet at home, so may not be able to update for a few days....