I'll recommend Keyclone strongly.
Also pick up the Jamba addon, post in the macro/addon forum.

Keyclone doesn't require scripts to run.
If there are any, they're in the background and you don't see them.

The FTL system for assists uses hotstrings, which is a bit complex.
Any other assist method is very simple.
Keyclone handles them all very well.

Within keyclone, on the maximizer tab, you can set regions for each instance of wow.
You can set a hotkey to switch from one region to another.
So if you have a large region for your main account and two smaller regions for the others...
Something like:
111122
111133
You can press your region 2 hotkey and they'll switch to look like this:
222211
222233

I personally have a hotkey per region within keyclone.
Then inside warcraft I have a switch macro of some sort, which I put on the same keybind.
So when I bring Region 3 to the front in keyclone, I press my toon 3 is in the lead macro in warcraft at the same time.





Simplest assist method is focus.
Every toon has a macro which sets the current lead toon to their focus.
So when toon 2 becomes lead, toons 1-3 all make toon 2 their focus.
Follow is something like:
/follow focus
Spells are like:
/assist [target=focus]
/cast spell name




A little more complex is using the party leader system.
This has the drawback of using two action bars, and paging between them.
It has the strength of leaving focus free, for pve crowd control or pvp burst macro's.

Basically, have two types of switch macro's per toon.
Every toon has a macro for themself which reads:
/changeactionbar 2
Every toon has a macro for the other toons (one per toon) which reads.
/changeactionbar 1
/script PromoteToLeader("Toon Name")

So when toon 1 becomes the leader, toon one presses the "bar 2" macro.
Toon 2 and 3 both press the Promote macro, except "Toon Name" is the Toon 1's name.

When toon 2 becomes leader, toon two presses the "bar 2" macro.
Toon 1 and 3 both press the Promote macro, except "Toon Name" is the name of Toon 2.

Every toon then has assist versions of their spells/abilities on bar 1.
Every toon has non assist versions on bar 2.
Bar 1 might be:
/assist [target=party1]
/cast Lightning Bolt
While on Bar 2, there is just Lightning Bolt dragged from the spell book.

When a toon is not the leader, they cast their ability/spell at the leader's target.
When a toon is the leader, they are on bar 2, so do not assist one of the followers.

In pvp you could create a macro:
/castsequence [target=focus] Elemental Mastery, Flame Shock, Lava Burst, Chain Lightning
Each toon will acquire Focus as their burst target, then target something else.
When the "Focus" is in the open.. mashing the macro will destroy it, without needing to target it.
Focus can be a powerful tool.




A lot of people like the target=targettarget system.

Every toon targets the party leader, who is the active/master character.
You need an assist macro for each ability. Something like:

/cast [target=targettarget] Lightning Bolt

Because your slaves are all targeting the master, their spell goes at the master's target.
I'm not totally familiar with this system, but the search feature will find it fairly quickly.




FTL is much more complex to set up.
It uses Keyclone's hotstrings feature.
There are a few guides for this assist method.
Once set up, its probably the strongest method.

The hotstrings have the non-active window assist the active window automatically.
So the slaves/secondaries assist the active/primary toon.
The active toon does not need to be the Focus, Target or Leader.
You just need to be in that window when you click/press the spell/ability.
Opposition have no idea who to target to take out the lead toon.

Like Focus, Target or Leader systems, any toon can lead as well as any other.
However, Focus is free, you can target anything with the toons and don't need to mess with Party Leader paging action bars.
If you can deal with the set up syntax, this is the system to use.





*Edit*
I have keyclone downloaded and extracted to four different folders.
They're basically the same, except I've named them...

Keyclone (2)
Keyclone (3)
Keyclone (4)
Keyclone (5)

The only difference between the four installs is their Maximizer settings.
When I want to play five accounts I get maximizer set up like this:
222333444555
222333444555
111111
111111
111111
This gives me a space on the screen for a browser or quest stacking guide.

When I want to run three accounts, it looks like:
11111112222
11111112222
11111113333
11111113333

Very basic. A shortcut to each.
Takes about 3 mins to set up maximizer, less once you're familiar with it.
Do it once, and its good forever.