Occasionally, this idea is not met and the group only has two or three players. At first glance, this might make running the game a little harder. However, its relatively easy to adapt the game to smaller group, especially if you follow a few simple tips.
First, I suggest allowing the group to begin at a higher level. As a general rule of thumb, I would make the level they start at equal to the number of players missing from the game. For example, I would have a three-player group begin at 2nd level (1st level +1 missing player). While I'd be starting the group at a higher level, I'd probably create and run adventures meant for a four-player group one or two levels lower than them (Example: For a three-player group at 2nd level, I'd use a 1st level adventure). This would help make the characters a little tougher, but still allow the adventures to be challenging.
Secondly, I'd probably make sure each character began the game with a potion of cure light wounds (especially if no one in the group wanted to play a healer). The reason for this should probably be obvious: to give them easy access to healing abilities. With that in mind, I'd suggest making healing potions readily available, or have a character invest in a wand at some point.
Finally, I'd probably advise the group to create a group of characters that would work well together and help cover any weaknesses they might possess due to the absence of a fourth player. For example, they could create a group consisting of a Druid with a bear companion, a Wizard, and a Bard. The druid's bear companion could take the missing martial role while the bard buffing the rest of the group with his spells and performances. You could also go the Bard/Paladin/Wizard route, having the Paladin be both the martial character and the healer thanks to his lay on hands ability.
While there are a few other ways to handle smaller groups within Pathfinder, these three tips are the first ones that came to mind. Hopefully these help the GMs running campaigns for smaller groups. If any has any more tips they'd give for running games for smaller groups, go ahead and leave them in the comments below. I'd love to hear them.