I would suggest doing 1 of 2 things (your choice)
A) reduce the temp of the room by maybe 2 or 3 degrees.
B) raise the heat plate up some
You could do both, but just not at the same time. Give them at least 24 hours of one change to see how they react before doing the other.
So: what was their keeping arrangements where you got them from (I know in the cellar, but temp, heat plate set up, etc.)
I did notice in one of your pic that all 3 were on TOP of the heat plate. That is a slightly warm space, but not like touching the underside of the heat plate. So, they probably do need some of it's warmth, but not to the level of being under it - unless it is the only thing they can all hop up on together????