You can go into wow and change keybinds.
If for example you don't use Numlock to toggle auto run on/off, then you can always have numlock on.
With it on, you can then take an action bar for the healer and put various heals on the bar.
You can then go into the wow keybinds and give each button with a spell a numpad keybind.

Now you have several heals on the numpad, which are keybound to do things on the healer.
Because the keybinds are not set on the other toon(s), the numpad does nothing for them.
This is the same as putting spells on 6-9 for the healer, but leaving them blank for the other(s).

There are three categories of toons when boxing.
Tanks do one thing.
Healers do another.
DPS of all sorts are in the last group.

I'd research "Click Castsequence" for your DPS needs.
If your tank is a paladin, you can bind a simple 96969 macro to the dps's spam key.
If you're not a pally tank, you'll probably want some keys for the tank which are exclusive to it.
It might be possible to make a tanking click castsequence too.

Anyway, you have a DPS and a Healer in your set up.
I'd probably go with click castsequences for both toons, set up as DPS.
But on the healer have theirs include the conditional [mod:alt] or [mod:ctrl], so if you spam the DPS key with that mod, the healer will also dps, but without the mod they do nothing on the DPS spam, which leaves them free to heal as needed.

Healer will likely have a key per toon in your group, which will be the toon specific heal.
I'd personally also have keys for their two or three most common heals, which assist off of the active toon.
You might have 1= Renew Healer; 2= Renew Mage; 4= Flash Heal Healer; 5= Flash Heal Mage.
And then 3= Renew Macro and 6= Flash Heal Macro.
And go with this format for your macro.
#show Renew
/cast [target=targettarget, help] Renew; [target=targettargettarget, help] Renew; [target=player] Renew

My macro assumes you target the Mage always with the priest.
So the macro casts renew on the mages target if that target is friendly.
If that target is not friendly it casts on the target of the mages target, which will likely be your Priest or Mage, or a tank if you group with a random tank for a harder quest.
You can use the assist method of choice instead of target=targettarget, works the same.