Was there an Iranian order for three Invincibles?

Why could Britain not take over an Iranian order for three Invincible-class ships and redirect carriers to Australia and Canada?

Because Iran had not reached a real three-ship Invincible order that Britain could seize or redirect.

The supposed Iranian order did not exist in the form required by the question. A ruler's wish for a ship is different from an approved, funded, and contracted procurement. Without that commitment, there is no ready-made three-carrier production block for Britain to inherit after the Iranian Revolution. A credible alternate acquisition path needs a project that had advanced far enough to redirect.