I do rear exhaust. Top mount also works. The primary benefit of the hybrid cards is the ability to dump the heat outside the case. Given you have a 280mm AIO for the I'd go rear, because I assume your top AIO is also exhaust, so that's three fans all exhausting in the top rear of the case. At that point your only concern is dust if the case the case ends up negative pressure, depending on the air those three fans move vs whatever you have on intake duty. For exhaust the only concern is that you're not pumping the hot air off your CPU tower cooler through the radiator, and since that's all going out the AIO, the only heat you're getting is off the VRMs.
Front mount isn't terrible unless you have a really small case with poor airflow. In most setups even if you have the radiator on intake, it's not going to heat up the case any significant amount. The only time I'd really get worked up over the hybrid on intake would be a super compact case where you have to go with a low profile like a NH-L9a or such and then mount the radiator literally right next to it, so you're blowing your GPU's hot air right onto a CPU cooler that is already fighting for its life. When I had two 980Ti hybrids in SLI in a midtower, all the gain was purely in moving the heat away from the immediate vicinity of the cards. Both rads exhausting out the top or intake on the front showed no difference.