Firstly you shouldnt just focus on opening the third eye and ignoring the others, as doing so is detrimental and can cause physical harm(such as headaches).
The best way to have an open chakra system and subsequently open the third eye is like previously suggested, meditation. When you meditate and wish to open your pathways you must work with the energy that lies in them. An easy way to do this is 'walk' the energy( or 'tingly' feeling) from your finger tips up your arms through your torso and down out your legs.
Doing this will give you a base feel on how your energy works and moves, and thus is a much easier and gradual introduction in working with opening the gates.
After that you can start to swirl and open your chakras. Make sure to work on each equally as they are all equally important and try to do so in order.
try this method it worked for me, i`ll give instructions step by step(generally find that giving instruction in that way helps the most):
1) Start meditating(sit or lie down comfortably,your breathing must be deep and slow(lower abdominal breathing), inhale on your nose slowly in such way that first your stomach raises then your chest, hold your breath for 2-3sec then exhale slowly on your mouth in revers method of inhaling(repeat until this type of breathing becomes natural/it may help to imagine a pipeline of sort for your breath that goes from your nose to your stomach and then back to your mouth)
2) Once you get breathing under control and relax then imagine a fountain(don`t over think it just a first fountain that come to mind, it should represent a direct line between your chakras), also it should represent you, then place it in a background/environment(again first thing that comes to mind would be best as it is most honest/it should represent your mind/spirit), and for the end try and feel as energy flows through fountain from bottom to top and out(truly feel it, so it may take some practice)
3) OK now this part is tricky, due to flow of energy you should be able to feel your third eye fairly easy and it should feel like your normal eyes(problem it was never or at least rarely used so its a bit weak)so only thing you need to do is "open it"(just imagine an eye opening and overlay it with where you feel your third eye being located), just be sure you don`t open your real eyes by accident its rather annoying as you have to restart every time you mess up.
After you can do it with your eyes closed try to do last part f step 3 with your eyes opened and without meditation.