Verified answer

Do you mean Pathfinder or Eberron? Those are two different systems and now a days they have classes unique to each. The answer also depends on what book you have access to.

You group seems to have the basic requirements of meat-shield, healer, trap-handler, and arcane-caster down flat. The only major requirement your party might NOT have is the party face and odds are pretty good someone in your group can cover that. You can also ask your mage what kind of arcane caster he is planning on being and go for an different kind.

If you're playing with the Pathfinder system, I can recommend the Bard. Pathfinder bards had public interaction, skills, and non-spell party buffing without even breaking stride. An archery bard is even capable of decent damage output. Another awesome pathfinder choice is to be a Summoner. You'll make the druid jealous with your eidolon and the mage jealous with your massive party buffs. Witches can also be entertaining. Pathfinder Monks can also be fun, but tricky to build.

In the 3.5 realm ask your DM about the Book of Nine Swords. If he allows it, the fighting characters in it can be lots of fun to play. (Though the monk-link class REQUIRES the special maneuver readying feat.)

If that's not allowed ask about the Player's Handbook 2 and become a Dragon Shaman. They are basically dragon themed Paladins, with buffing auras and natural breath weapons instead of spells.

If your DM says PHB1 only you need to look at the weapons and techniques your party uses. If you are mostly melee, break out a ranger and take care of survival and death-by-arrow. If you are mostly ranged, barbarians and paladins make good melee companions to rouges. Since you are starting at level 3 you can also play a monk, but it will be another level or two before that class really gets into its own.

Pathfinder Eberron

It really depends on how the Fighter is going to be built up to really get a idea of what to be. Most clerics are not really front row fighters since they can not heal effectively on the front line. So as it stands the only real front line person is the fighter. Is the fighter going for more offense or defense? If they are going offense then maybe grab a fighter and put him more defense wise, maybe work for the Stalwart Defender prestige class. Make him a dwarf fighter with a heavy shield to support your second tier fighters such as the cleric and rogue and keep opponents off the mage and druid. If the fighter is more defense or balanced maybe go with a Barbarian or even a ranger and work on the two weapon fighting style and not ranged part. Either way the best feat to really get first would be the quick draw feat. If you are going with the Stalwart Defender route go with a Die Hard Feat and maybe a combat expertise feat and work on the improved Sunder and Improved Disarm feats as well as what is required for that prestige class. Any feats involved with the shields is also a good thing to have. For a more offensive character go for the improved initiative feat after the quick draw. After that anything that will work for the chosen weapons of your character. For barbarian got for a two handed weapon with the feats power attack, cleave, and greater cleave feats. For two weapon ranger you will of course need all the improved two weapon focus feats as well as stuff like two weapon rend and defense to increase your damage on your off hand weapon as well as increase your defense while having two weapons.

Play a Half-orc Barbarian Min/maxed for accounting.

Stats

Str 10

Dex 10

Con 8

Int 15

Wis 17

Cha 15

Also don't spend the points for literacy. Its never worth it.

A basic monk would fit in nicely with that group.

Be another cleric. You can never have too many clerics.