I've been diagnosed with postural orthostatic tachycardia syndrome, m.E fibromyalgia & eds3, what's the best way to treat and manage it all? Exhausted!

Core stabilization. People with connective tissue disorders and loose ligaments often have muscular pain from the large muscle groups working over time to maintain posture.Best is to slowly build core strength and stability muscles (slow twitch) so the larger (fast twitch) are used as they are supposed to (movements versus posture). A good physical therapist can help with this- gives endurance, prevents fatigue.
POTS . Pots can be especially challenging, and conventional medicine has little to offer people with this condition. My healthtap colleagues may laugh, but the case of someone successfully treated with this problem was presented by california center for homeopathic education this past weekend. http://cchomeopathic.com/ with a well-taken case, there was profound cure. Not an easy process though.
POTS. Pots often has people exhausted. There are a host of diet, exercise, medications which have been advised and tried. Might ask your doc to refer you to someone who specializes in this if you aren't responding to treatment as yet. Often patients have to go through many trials to find something that helps.