they all seem like the same damn thing to me. what am I missing. its still my same workout in three different places, each has there own made up little algorithms to make the numbers different and magically make you seem faster or that you worked harder. what the hell am I missing here?
(before I go to far, I don;t pay for strava or training peaks, use the freemium versions)
Which platform do you like best? I prefer Strava myself so that is the one I use. If you are into the whole segment deal I believe Strava is the king of that realm. There will always be multiple offerings of a service that is in demand.
each has there own made up little algorithms
Actually, they don’t.*
(*Yes, Strava skates around TrainingPeaks’ trademarks by subtlety changing the calculations and using a different name, but it’s really the same thing. Garmin, OTOH, uses the same algorithms and acknowledges TrainingPeaks’ trademarks…which is as it should be.)
Training Peaks (free) is fairly limited. For me the biggest draw for TP is the paid features of scheduling and data analysis.
Golden Cheetah is a free alternative to TP but it is a bit more difficult to learn and work with (imo).
Strava goes with more of a social approach.
GC - generally not much there, but people like it because it transfers to other services. Personally, I just use https://tapiriik.com/ to transfer between services and skip GC.
It very much depends what you want out of these services.
If you just want to look at a GPS trace and see some summary stats then any of them will do, but to briefly summarise the strengths and weaknesses of each:
Strava: *Pros *- best social networking, segments can be fun/motivational. *Cons *- Limited and somewhat subversive statistics, only properly supports bike and run activities.
Garmin Connect: *Pros *- solid basic-level service, supports loads of activity types (inc. multisport), excellent integration with Garmin devices. *Cons *- Limited (but better than Strava) stats and rubbish graphs with little customisation.
**Training Peaks: ***Pros *- loads of unique stats (TSS, CTL, ATL etc etc), highly customisable graphs that can show you almost any stat or trend you can think of, great long-term planning/tracking features. Cons - have to use premium for most features which costs $$$, interface is a bit industrial.
I have my data in the free versions of Strava and TrainingPeaks and in Garmin Connect too because its free and automatic to do so but, I use the free Strava version almost exclusively for basic day to day tracking and as my de facto training log and I use Golden Cheetah for analysis since I’m too cheap to pay for Strava Premium or the full version of TrainingPeaks 
Strava, for me, is 50-50 social and ride analysis/workout log. The social part of Strava is huge for a lot of people and really is their differentiator and big hook. For me, (and I know I am not alone) all my cycling friends are on there and I enjoy keeping up with what everyone is doing. I literally look at it at least once a day to just see what everyone else is doing and that alone, for me, makes it worth using it. The basic ride data the free version gives me is much more extensive than the free TrainingPeaks, both on the desktop and on the mobile app. My training routes are littered with Strava segments, and I have made a few of my own, so basically any training ride I do has all the hard efforts I care about already broken out separately.
I have recently started using Golden Cheetah for deeper analysis. Its kind of clunky in some ways but it has the full version of almost any analysis tools anyone would need and its free.
I have never figured out any need at all for Garmin Connect other than a parking place for my data to be automatically uploaded to Strava (and TrainingPeaks).