3 thermometers give 3 different temps!

Using one thermometer right after another, the Meijer brand ear thermometer says 97.0, an old fashioned mercury thermometer says 97.8. (When I went back to the Meijer brand it said 97.0 again and I was not moving around a lot or anything.) A digital oral thermometer said 98.4.

What's a gal to do to get an accurate reading? I am trying to induce a fever in a sauna as I have read that can be very, very helpful to the immunological system. (My head is not in the sauna, but outside.) However, I don't want to go too high, not above 102 degrees. I also want to know how well my thyroid helps really work. I take Thyrogold and kelp.