(sorry no WE handy)
basically exactly what u said is the answer. every 30 seconds, a trigger runs, counts the footman and gives an arithmatic equation of 3 x footman gold, then adds it to the players gold.
Event: Periodic timer of 30 seconds
Condition: N/A
Actions:
set IntVar = 0
unit group - select all units of type "footman" matching owned by player X and do:
---set IntVar = IntVar + 1
player resource - set gold to (current gold + "IntVar")
****One for each player to be taxed or make a loop by player number
****leeks a unit group check things that leek sticky