Hi Matt,
The X system is a convenient way of expressing the diameter of a material in thousandths of an inch.
The rule is 11 minus the X number gives you the diameter.
So, in the case of 3x
11-3 =7
diameter of 3x will be 0.07". I tend to use the X system but if I ever need to explicitly express the diameter of tippet I will quote in mm.
Forget breaking strain, diameter is far more important. For example, you will find that 15lb Maxima Chameleon turns over 1.5" tubes O.K but 15lb Orvis Super Strong will not. That is because 15lb Maxima has a diameter of 0.37mm and 15lb Super Strong has a diameter of 0.28mm. If using Super Strong and buying by breaking strain, you would need to buy 25lb, this is 0.38mm and will handle tubes just fine.
Is that making sense?
Ideally, the size / mass / bulk of the fly will dictate tippet diameter, a size 20 dry will be fished on 6 or 7x if it is to fish well. Don't expect that tippet to hold a 6lb Don trout if you are fishing it on a fast 10' #7 rod, on a 4wt it should hold just fine.
Personal preference when trout fishing is for copolymer tapered leaders and then tippet of fluorocarbon, I am using Mirage Max knot Strength and have found it to be excellent but, as with all fluoro, you need to knot it very carefully. I have found that no fluorocarbon material likes 4 turn water knots or surgeons loops.
Cheers
Ben