I had a recent/same problem when I went from only one camera to four. One big change you can make is turning off the image channel from updating. My guess is the binding can send data faster then what your Pi and openhab can process when you have more then 3 cameras. Openhab seems to send the picture data down the event bus and this is the bottleneck which is not my coding. Also it appears to leak memory. Good news is I added a feature to turn off sending the data to openhabs frame work and these problems went away. No memory leaks, it halved the cpu usage and no signs of OOME heap space errors. The readme has details on turning off the UPDATE_IMAGE=false and how to get a picture using urls. I also did a howto post.
I would also plan on upgrading your Pi to something with at least 2gb of ram if not more. I use odroid c2 which has 2gb ram and had five cameras running for weeks with a heap size of 768 from memory which the Pi3 won’t handle.
Try turning off image first and then maybe one less camera till you upgrade.