As CapnZapp says this is (yet another) situation where a DM needs to make a decision and stick to it.
Personally I run it from the point of view that a single action (or power) is one attack for
special triggers and effects. (for my own group
Divine Challenge and
In Defence of Order are probably the only things I currently class as a special triggers)
example.

Tripple claw (standard, at will)
Dragon makes 3 claw attacks
The dragon in question is using a single action to use one power, that power just happens to involve 3 attacks, but I key special triggers and effects from the action (Tripple Claw), not the mechanics (3 claw attacks).
Example to show things arn't necessarily that straight forward.
For divine challenge, if the paladin was one of the three targets (either state the targets when declaring the attack or target the Paladin first) then its divine challenge damage would not occur. If the Paladin is not included in the attack then the divine challenge would activate like normal. (simple enough and follows my stated logic)
But, in the case of divine challenge I actually allow the -2 penalty (marked) to attack rolls against targets other than the paladin when I am making multiple attacks from a single power (the powers mechanic is 3 swipes with its claws). If it was an area attack I wouldn't. (this isn't really following straight forward logic, it is just the way I do it, and attempts to give something back "balance wise" for reducing the occurance of the divine power damage)
My view on your examples:
Does a multi-attack action, like a dragon's triple-claw attack, count as a single attack action for the sake of run-away interrupts or hospitaler's blessing heals.
Or if a hydra targets a ranger with a multi-headed bite attack, and the ranger has an immediate reaction, will that take place on the first attack or the last?
Hospitalers Blessing, Tripple Claw all attacks at a single target other than the Paladin is counted as 1 attack. I would probably say that each creature it targetted with this
power would get the Blessing effect, but only once each per power use.
If the paladin was targetted by one of the 3 claw attacks then I would say that the Blessing is not activated.
Interrupt action - run away, I would count each of the 3 attacks seperately (but each PC can only use 1 interrupt action anyway). In this case the first attack would fail and the PC would probably be out of the creatures reach so it would make any remaining attacks against other targets.
Immediate reactions, again I would count each of the 3 attacks seperately, like the above example.