For starters, the each skill has specific functions that are called out in their individual descrptions:
Bluff:
-Feint in Combat
-Create a Diversion to Hide
-Deliver a Secret Message
Diplomacy:
-Influencing NPC attitudes
Intimidate:
-Demoralize an Opponent
Beyond those uses, their more general functions are generally left in the hands of the DM, with the skill descriptions serving as guidelines for use:
Bluff
Used for when you are short on time and need to momentarily convince an NPC to go along with your suggestion. These comprise situations where you engage in fast-talking someone into giving you something that you want, but will generally not stand up to any kind of scrutiny. They key with this use is the length of time involved. It's done very quickly, and can fall apart very quickly.
Diplomacy
This is for extended and in-depth negotiations where you are attempting to persuade your target to see things from your point of view. The shortest amount of time you need for an attempt is one minute, with penalties applied to the check for any shorter length of time spent. Note that nothing in the rules requires you need not be honest in your negotiations, you are simply spending a greater amount of time on them. A commonly missed rule here is that during extended negotiations Diplomacy is an opposed check, with the side that rolls higher succeeding. This is also the case when two opposing sides are making a case to a third party. Two other commonly overlooked rules are a) the DM is within his purview to arbitrarily establish a limit for how far any given NPC's attitude might be swayed with regards to a player character, and b) NPC's are never required to act as their new attitude dictates. Persuading a prison guard to helpful does not mean that prison guard will unlock your cell and let you out, though he certainly might try to sneak you some extra food at dinner. Similarly, persuading the head of an evil empire into helpful does not mean he won't still try to kill you for opposing his plans. He might just feel really bad about it afterwards.
Intimidate
Beyond it's specific use in combat, this skill functions exactly like Diplomacy, except with a hard limit on the duration of the results. Intimidating someone immediately sets their attitude to friendly, with all of the above limitations that entails. But after you leave and the target comes to their senses (again remembering that they are not required to attack you on sight the next time they see you, despite being hostile).