Advantages.

1. It can get really weird
2. Someone can go and make a pot of tea without interrupting the proceedings
3. There's always a hand or mouth free when you need one
4. Hotel rooms split 3 ways are only £13.50
5. You get to watch your best friends making love
6. You get to get watched making love
7. Simultaneous enjoying intercourse and oral sex has to be experienced to be believed
8. You get strange looks when you all go out dancing
9. You get really strange looks when you all go out comparison shopping for condoms
10. Enough people to play gin rummy if things don't work out
11. You can safely check your friends for lesbian tendencies without actually doing anything about it
12. Calling out the wrong name during climax isn't as much of a problem, the "wrong name" is probably the one on your left
13. Three-person showers are fantastic
14. Three-person naked belly laughs are even better
15. Three-person kisses are best of all.

(11. Might also be considered a disadvantage)

Disadvantages:   

1. It can get really weird
2. Tougher for three people to decide on pizza toppings
3. Simultaneous orgasms are even trickier to pull off
4. You may get paranoid thoughts that while you're in the bathroom; the other two are giggling over your saggy tits
5. Trying to find safe places to put your elbows/feet/knees/head/etc
6. You get to find out what kind of really sick things your friends like
7. Queen-sized beds are suddenly smaller than you remember.
8. Trying to fit 3 names in the little heart when drawing on your notebooks
9. Morning breath multiplied by 3
10. You might discover weird tendencies you didn't know you had. 
11. You might discover lesbian tendencies in one of your friends you didn't suspect.
12. You have the option of wrecking twice the normal number of relationships
13. The odds of boyfriends/spouses walking in on you triple
14. Sorting clothes when it is time to go assumes comical proportions
15. Now there are two wet spots to avoid.

(11. Might also be considered an advantage)