I was wondering how indenting recursively more and more lines of poetry following a custom rule. For instance

Let say we have:

OF Mans First Disobedience, and the Fruit
Of that Forbidden Tree, whose mortal tast
Brought Death into the World, and all our woe,
With loss of Eden, till one greater Man
Restore us, and regain the blissful Seat,
Sing Heav'nly Muse, that on the secret top
Of Oreb, or of Sinai, didst inspire
That Shepherd, who first taught the chosen Seed,
In the Beginning how the Heav'ns and Earth
Rose out of Chaos: Or if Sion Hill
Delight thee more, and Siloa's Brook that flow'd
Fast by the Oracle of God; I thence
Invoke thy aid to my adventrous Song,
That with no middle flight intends to soar
Above th' Aonian Mount, while it pursues
Things unattempted yet in Prose or Rhime.
And chiefly Thou O Spirit, that dost prefer
Before all Temples th' upright heart and pure,
Instruct me, for Thou know'st; Thou from the first
Wast present, and with mighty wings outspread.

And we want to indent it, recursively, adding 3 spaces to any three lines after the first, in this way

OF Mans First Disobedience, and the Fruit
   Of that Forbidden Tree, whose mortal tast
   Brought Death into the World, and all our woe,
   With loss of Eden, till one greater Man
Restore us, and regain the blissful Seat,
   Sing Heav'nly Muse, that on the secret top
   Of Oreb, or of Sinai, didst inspire
   That Shepherd, who first taught the chosen Seed,
In the Beginning how the Heav'ns and Earth
   Rose out of Chaos: Or if Sion Hill 
   Delight thee more, and Siloa's Brook that flow'd
   Fast by the Oracle of God; I thence
Invoke thy aid to my adventrous Song,
   That with no middle flight intends to soar
   Above th' Aonian Mount, while it pursues
   Things unattempted yet in Prose or Rhime.
And chiefly Thou O Spirit, that dost prefer
   Before all Temples th' upright heart and pure,
   Instruct me, for Thou know'st; Thou from the first
   Wast present, and with mighty wings outspread

What can be the simplest way to achieve this goal?