I did try connecting the tuners to a hub, but not a powered one. I'm reluctant to spend more money on one in order to test this, although I may be able to put my hands on one.
Try to get one powered hub at least for testing, no need to be USB3, one old could be good too. Just to check it what's happen if the tuners doesn't draw the power from RPi. I'm not saying that the problem can't be something else, but if with one or two (exactly the same) tuner it's working well, should be useful to exclude the power issue.