Two friends and I just ran a three-GM adventure with a group of seven first time players. Though having a plurality of GMs could potentially be a "too many cooks" scenario, if managed properly it can be a great success. I'll try and explain why, and how to ensure that it doesn't degenerate into a tavern brawl...
The adventure we ran was based on the Harry Potter stories (OK it's not high fantasy but it provides a good introduction for first-time players because it gives them a setting with which they're familiar). The characters were new students at Hogwarts School of Wizardry and Witchcraft and soon found themselves in the middle of a crisis when an ancient legend about the Sword in the Stone started to affect them. The system we used was a mix between LARP and freeform magic rules (see CaptainPenguin's thread
). Though there was die rolling for combat this was kept to a minimum.
1. Three GMs takes the pressure off: We'd discussed the general plot outline between the three of us and each taken sections to design and run. Would this not lead to a lack continuity and coherence? Not at all, it made perfect sense. Often new sections of an adventure involve a new NPC: one moment they're being shown around the school by a cracked old janitor, the next they're escaping from a dormitory accompanied by a house-elf who's supposed to be chaperoning them, then they find themselves face-to-face with a decendent of a famous wizard in his forest hut. The transitions between GMs are actually quite natural.
2. Contrary to expectations, it doesn't destroy the flexibility you get with one GM. While one GM runs his section the other two can be elsewhere discussing how to modify their plans based on what the PCs are doing. They can let the other GM know later.
3. Special effects and props are hard to incorporate into traditional tabletop games, but when we had magical duels taking place one of the GMs would be there to work the strobe light (which we got cheap from Argos). Music becomes easier to use if someone's there to operate the CD player: if it gets overpowering they're there to turn it off. Also, the NPCs could be costumed: while not busy a GM can go and get changed. This caused much hilarity in the case of Hagrid, the school's resident fat, hairy beast.
4. Changing the face with whom the PCs are dealing can make it more interesting for them. A session with only one GM could get monotonous. This way makes it feel more like the story is actually developing. I would never usually consider running a game with a group of seven first time players: they'd lose concentration after half-an-hour. With three people to control them you can keep them in order: with two or three pairs of eyes you can make a lot more eye contact and break up small conversation groups. It will also make them feel a lot more involved: one GM cannot divide his attention equally between so many people, so someone ends up feeling left out.
5. There is potential for the groups to split up. How often do your PCs say "Well there are two paths so me and Jimmy are going this way and you two can take the other path and see who gets there first," and you think "Argh! Logistical nightmare!". With two GMs to control the two groups and a third to communicate between them it suddenly becomes a lot easier.
6. It makes dinner a lot easier. Two GM were cooking while the other introduced the players to the school. That meant we could offer them an experience like a Formal Hall (except it was in my friend's kitchen).
7. It enhances creativity. Three minds can overcome GMs' block more easily than one. Although there can be artistic differences these do not lead to unpleasantness if everyone acts rationally. I know if I'd have been the sole GM in charge we'd never have had Pierre and The Duke, the two talking paintings whose gossip gave the PCs the vital clue to what was going on (it was actually my two fellow GMs sitting on top of a large wardrobe looking through picture frames).
8. The only proviso is that you must not become jealous of the other GMs having power. They are not minions, you are a triumvirate. If at every point you are trying to take centre stage from them then it would become nasty. This is why designating sections of the adventure is useful.
The biggest difficulty is getting three GMs together to run the game, but believe me it's worth a try!