I've been tri-boxing a 70 Prot warrior (Main), a 70 Shadow Priest and a 70 Enhancement Shaman for a while now. These characters have been levelled separately, and was only quite recently teamed.

I do not run instances with this trio, I only group them to do dailies and quests in zones that I've not cleared. Beyond doing dailies and completing quests in the various zones, I do not intend to tri-box these characters much.

For this purpose I do not need many macros on the characters, generally this what I have:

1. A invite macro on my main (unbound)
2. A Charge macro on my warrior, bound to ALT-1
3. A Tanking macro on the warrior, bound to ALT-2
4. DPS Macros (castsequence) on Priest and Shaman, bound to ALT-3
5. Renew macro for Priest on ALT-4 (only heals tank)
6. Greater Heal macro on Priest on ALT-5 (only heals tank)
7. A Shield macro on Priest on ALT-6 (only shields tank)
8. A follow macro on ALT-7
9. A Flying mount macro bound to ALT-9
10. A Prayer of Fortitude macro on ALT-0
11. A Land mount macro bound to ALT-=

All I really need to to is hold down the ALT key and mash 2 and 3

(I do not have the machine available at the moment so I can't give you the exact macro's)

The Charge macro swithces stances and uses any of the charge abilities in either stance, depending on what is available.

The Tanking macro is very simple... /cast Devastate. I could probably weave in more abilities, but for now it does what I need, and the TPS is good enough to keep ahead of the 2 other DPS'ing

The DPS macro's are not very complex either, they only consists of a /assist <Main name> and a /castsequence <Ability1>,<Ability2> etc.

That's really all I need to grind dailies and quests, as I do not need to worry about being attacked by the other faction on a PvE server. I could certainly expand a lot on macro usage, but I see no need as what I have serves the purpose for this group.
One of the drawbacks I have with my setup is healing. I'm normally only able to heal the tank easily, so if any of my other characters die, the other 2 may be seriously disadvantaged, this is generally not a problem when questing, but would become a problem in instances. This does not bother me much as I do not intend to bring all 3 into instances at the same time.

For my 3 lvl 8 mages, I use (or will use) many more macro's than I use now, as almost every ability will be macro'ed on the 2 'passive' characters.

I also intend to level another trio of different classes at some time. The trio I got in mind is currently a lvl 8 Paladin, a lvl 20 warlock and a lvl 40 mage that I never really felt like levelling solo