step 1:
Use trinity bars

Step 2:
Make the following macro on TrinityActionButton60 (or another action button)
/target [target=focus,help][target=toon1,help,nodead] [target=toon2,help,nodead] [target=toon3,help,nodead] [target=toon1] ( you have to modify this for each of your toons to set the proper order for main)

Step 3:
Make 5 different macros for all your toons to set a friendly focus, this way you avoid the focus set macro if you have a keybinding for each of them

Step 4:
Make all your attack macros like:
/click TrinityActionButton60
/assist
/cast Lightning Bolt

Result:

You will assist focus if it is friendly. So you can use your focus to select main if you wish. If you dont have focus or a hostile focus you will assist in the selected order. Making your focus free for cc and other purposes.