The top and bottom pics might be plum leaf gall mite, but this usually shows itself on the upper surface, as in the lower pic, rather than underneath. This assumes they're not little green bugs with legs that move, it's hard to see properly in the pic. No treatment available, but doesn't usually harm the tree anyway. As for the second pic, if it was on hydrangea I'd be telling you to examine the stems, branches and trunk for signs of scale insect, so that might be worth doing anyway - otherwise, just looks like whitefly.