Hank,
Those amps run very cool and seem to be strong enough to stack three...
Strong enough yes, but "cool" is relative. I am not sure what sort of speakers you are powering, but after 2 hours of high SPL listening on my Maggies, the monos are almost too hot to touch on the heat sinks and the case itself is mighty warm.
I used to think I needed to add a fireplace to my house, but since I added the maggies with Odyssey monos, I have ample extra heat availabe.
I asked Klaus about the vibrapods ratings and he explained that the feet he put under the amps were rated for weights ranging from one amp to well over the weight of two (I plan to bi-amp soon, so they will get stacked on both sides). I am not sure about the weight of the HT3 - if the total weight exceeds the rating of the feet under one unit, I'd get the next higher rated vbibrapods for the bottom unit. Check with Klaus for the exact rating. I only needed to know about the weight of two Xtremes.
Peter