OP, get fitted for and ride all three -- for extended periods if at all possible -- on the kinds of surfaces/in the kinds of conditions you most anticipate using the bike (you are intending to spend some decent money; a good bike shop should be willing to help you make an informed decision). One should 'speak' to you more than the other two (the three are distinctly different beasts, notwithstanding that two are called 'FX').

Three thoughts:

1. If you prefer the handling of the rigid bikes (you may not), but want a little more 'cush' to your ride, remember that the 7.6FX will take tires up to at least 35, and possibly 38 width. An easy and relatively inexpensive change; wider tires can be run at lower pressure with no increase in rolling resistance. Result is more suspension on rough surfaces. The 7.7 limits you to 28s.

2. If you prefer the handling/feel of the 8.6DS, don't be put off by all the talk one hears on this board and elsewhere as to how "suspension is totally useless/unnecessary unless you are really mountainbiking" etc. blah blah. Some people simply do prefer active suspension, even on the road; seems reasonable to me. If that's what you prefer, that's what you prefer. What matters is whether or not the bike in question has a decent fork; the 8.6DS does. The Suntour fork on that bike has a proper coil spring and proper hydraulic damping/lockout. I find, even at 62, that I'm fine with a good, light rigid frame/fork with wider tires (32s, in my case) for mixed surfaces, but that's me, not you.

3. If you really, really like the 7.7FX, and end up not concerned about limitations on tire width or load-carrying, go for it, and forget any nonsense about carbon 'asploding' or being otherwise ultra-fragile; it isn't.