You can look across the lower bout to see if it is bulging upward, (or check it gently with a straight edge). If it is noticeably raised in the middle, (and was not built that way), then you have too much moisture. You can also look to see if the corners of the bridge are lifting. Wood can move a little, but you don't want it to move a lot. I would keep them in their cases when it is humid and if you get a dryer spell, you could leave everything open for a while to dry out. The worst thing for an instrument is sudden changes, and it doesn't sound like your humidity drops all that much. Too much moisture will raise the action and can cause permanent damage and cause the corners of the bridge to lift if the humidity is extreme, but it doesn't sound like you have that extreme.
Three on order all at the same time, cool. That should be fun.
A dehumidifier would probably make the room more comfortable. If you run it at 55% your energy costs shouldn't be very high and your instruments should be happy.