Agreeing with the above. HP Fire (and Grass; plz mention that as well!) is very useful for reliably slamming many of the mons that are usually sent in to eat up the Volt Switch.
That said, I really think that Heal Bell should be the main--and only--move in the third slot, and Focus Blast slashed with the additional offensive options. It's rare that Ampharos will be using anything but Volt Switch or Dragon Pulse. Both are generally safe enough to haphazardly spam (especially Volt Switch, thanks to Mold Breaker), and riding on Focus Blast is always a risky gamble (e.g, do you want to go for the 2HKO on that Nattorei, or risk it getting up a couple layers of hazards?). I've found that the potential utility of Heal Bell is almost always too good to give up for BO / rain, and Ampharos has plenty of chances to get in and use it. Heal Bell adds a pretty strong feeling of safety to the teams Ampharos is used on; and given that Volt Switch tends to bait in slow, defensively oriented Pokemon (Dragon Pulse wards off the more aggressive switch-ins, namely Garchomp and Lando), maintaining momentum shouldn't be too difficult, so dropping Focus Blast isn't too much of a loss.
Additionally, what do you think about a spread of: 124 HP / 252 SpA / 132 Spe for the tank set? It outspeeds neutral min Speed TTar, and, far more importantly, 0-4 Spe Clefable. The HP reduction usually doesn't matter much because of the set having a more offensive intent, and it puts me, at least, at ease not having to tiptoe around one of the best Dragon checks in the tier.
Lastly, I think that Agility should be revisited and possibly given a set. Taking into account that Modest 252 +2 Mega Ampharos outspeeds max Greninja by one point, and it has (in the words of AG) "the power of Specs Jolteon / LO Thundurus" (roughly), cleaning offensive teams is definitely a possibility. Moreover, given Mega Amph's superb defensive and offensive capabilities, finding a chance to set up can be very, very easy. My biggest problem with Agility is losing out on one of Ampharos's most powerful niches: Volt Switch. Agility exchanges the ability to maintain momentum for a chance to punish the switches it causes, and potentially clean very specific team archetypes; any team that isn't offensive usually has a consistent Mega Amph check that doesn't care much about getting outsped. On the brighter side of things, Agilipharos doesn't mind priority as much as many other Speed boosters, and is immune to Thunder Wave (Thund-I), which makes it pretty difficult to stop if it isn't being walled outright. Of course, Scarfers are also problematic, but it's not too difficult to wear them down (or KO them on the switch). Furthermore, there's atypically no indication that Ampharos is running Agility, so the opposing player doesn't have any incentive to keep their Scarf alive for the sole purpose of keeping Ampharos in check. Something along the lines of:
name: Agility
move 1: Agility
move 2: Thunderbolt
move 3: Dragon Pulse
move 4: Focus Blast
ability:Static
item: Ampharosite
evs: 4 HP / 252 SpA / 252 Spe
nature: Modest
with a mention of Timid nature / other coverage moves sounds appropriate. It's certainly not better than the two defensive sets--far from, in fact--but it's capable in OU, and imo, should be given more love.
