but it is accepting RGB and HSV (three channel) images.
No, regionprops3 is for volumes exclusively. If you pass it a 2D colour image (MxNx3) it will be interpreted as a (very thin, only 3 layers) 3D label matrix.Or if you're talking about colour volumes (MxNx3xP), these can't be passed to regionprops3 directly, you'll need to convert to greyscale and then binarise the volume.
If you are working with 2D images, you need to use regionprops not regionprops3. Again, a colour image must be binarised (or labeled).