For opening a chakra is necessary years meditation on one chakra only.
But, by my opinion is much better then Third eye - chakra Ajna - opening chakra Anahata.
This path is outlined in book The secret of the golden flower, translantion by Richard Willhelm. Is free on net.
About three months meditation on chakra Ajna (in this book a Heavenly heart) and then a years meditation on Anahata, until is chakra open. Meditation on chakra Anahata you could done in common life too, and before you sleep. By this way is possible accelerate result. But on Anahata only. On Ajna not.