I've been one of the two DM's for the same group of players for about 4 years now, and I will admit that I've gotten extremely used to this group. As of late, clashing work schedules and personal problems have forced most of the "regulars" to either stop gaming or switch nights. One of the players had another group that he occasionally games with and invited me to run a game for them, and i agreed. Here's the problem; I'm used to running games with six to eight players, and this group consists of three or four (depending on the week, one player can only make it about half of the time). any advice for adjusting to the smaller group?

Prepare for more encounters and battles. Smaller group means bigger play time per person, quicker battle resolution, and faster decisions.

I understand the idea about preparing more encounters (I usually have 3 or more sessions worth planned in advance, depending on the group's choices), but I'm a little more concerned with toning down battles. the group is only comprised of three steady players, and they are playing a Kobold Cleric, a Ratfolk Rogue and a Ratfolk Gunslinger. I'm a relatively experienced DM, but i'm not used to teams that don't have frontline fighters. any adjustments i should make to combat?

As an experienced GM, im not going to tell you how to run your games, and i'm certain you already know all about calculating APL to create encounters of appropriate CR.

My advice? I'd say go with encounters you think the group should be able to handle easily. Take note as to whether or not they struggle, and throttle the rest of your encounters accordingly. Take note if Undead are especially difficult for them to deal with. How about against a regular front line fighter? Oh, and make sure they dont come across any opposing adventuring parties. Because you know its 2 rats and kobold according to the games ives GM'd that basis for murder and mass looting. God forbid anything those 3 wear shines. :)

Anyways im rambling,
TL;DR
Take it easy so you dont start with a TPK, and make it harder as you go.
P.S
Good photo choice. :)

Definitely.

If a party has no frontline fighters, scale down the str/damage bonuses and hp of melee opponents.

If a party has no dedicated healer, have an NPC or poof in some kind of healing fountain or add potions/wand of cure light to the loot table.

If a party has no magic users, consider dropping SR if it exists, make it easier for your melee fighters to deal with ranged spells.

Basically, know your new group, know your regulars, and be able to compensate for imbalance with ideas. Thinking on the fly makes for much better storytelling.

Between encounters, think ahead. (Ok I set up this fight for the mage's benefit, but now he's not here.) Make the BBEG an illusion that taunts the fighters and escapes because he's bored.

As far as scaling CR and things of that nature, use good sense. If your entire party is basically APL 8, and your throng of 10 kobold wizards is CR 8, but only half your party shows up, try to compensate by halving the amount of wizards. If the party looks like they're overwhelmed, make two of those wizards illusions, phantasms, or a clever mirror image and wipe them off the board. If they are underwhelmed, (pop) in comes the reinforcements via the invisible guy no one saw in the corner.

The DM's best tool is his brain. Don't feel too bound by the rules or amount of anything or try to drive the PC's along a linear path. They will surprise you with resourcefulness.