Hygrotherm vs. Exo Terra digital meter accuracy?

[Yarjammer]

I am trying to figure out which of my temperature and/or humidity meters are the most accurate. In my 18x18x18 terrarium I have an Exo Terra digital temp, Zoo Med Hygrotherm, and an Exo Terra temp/humidity meter. They are placed in that order across the width of the enclosure about 4" from the back wall. The problem is that the Exo Terra's are in agreement and are usually 3 degrees cooler than the Hygrotherm. The humidity is off by up to five percent. Since I'm using the Hygrotherm for climate control I am trying to figure out if I need to adjust its settings based on the indicated difference (i.e. A correction factor) to keep the settings in line with my Pacman's habitat needs.