I agree with J1M, structure helps you improve, otherwise you're just training to run 6 miles at your comfy pace. Just make one run your long one (and make it slow - so you can chat nice and comfortably - to an imaginary running partner!). Increase length it by 1-2 miles per week til you get up to 10, 11, 12, 13, 14 or however high you're going. (My longest training runs for HM are around 10/11 miles). This long slow run gives you endurance and mental stamina. Keep another run at 6 miles (make this a steady, mid-pace run, so you can talk in short sentences). You could play around with the last run, add in some faster paced miles as intervals; if you do I'd be tempted to make it shorter, say 30 mins intervals or a tempo run (just sustainable pace, where you can praps only utter a word or two).
You definitely can do a HM training on 3 runs per week, I am living proof! I tried going up to 4 and got ITB problems. Now I run 3 times and cross-train once. But you do need to use 3 runs wisely to make sure you get some endurance and speed in your legs.
BTW your 10 miler is really well positioned to give you a good idea of how you're doing and how to pace your HM. Use it as one of your long runs. Good planning! And don't forget to have a wee taper before your HM; I definitely recommend it. Good luck! Where are yr races?