Those three needed better HA, if I remind correctly they are still unreleased so there is a chance to get better HA.
All three would need Analytic for example.
Otherwise to each its own.
> Regice
Moves: Freeze Dry, Mirror Coat, Haze, Dazzling Gleam, Endeavor, Heavy Slam
HA: Analytic, Pressure, Regenerator, Heatproof
> Registeel
Moves: Counter, Mirror Coat, Mirror Shot, Discharge, Endeavor, Heavy Slam
HA: Analytic, Pressure, Filter, Bulletproof, Regenerator
> Regirock
Moves: Endeavor, Rock Blast, Heavy Slam, Counter
HA: Analytic, Pressure, Solid Rock, Regenerator
Heatproof Regice is because ofbthe dex entry, yet the other abilities help it more. Registeel's HA are all themed and fitting competitively. Regirock has also self explicative HA. All of them could also support and benefit from Analytic and Pressure that are very natutal on them, Regenerator is more about an inorganic recycling of their primal type elements. All of these moves and ability alone would be worth of use and some usage shift.
The current HA still stand because from Gen1 to Gen3 all the minor trios have in common abilities that are strictly related to their body, either absorbing and recovery ones or status inducing.
