I agree, all of them are almost certainly horse-harness buckles, especially the first two. More specifically, they are from the heavy harness of a wagon-pulling horse. Note that your second buckle is extra-thick... in order for two thick leather straps to pass through it.
Relic-diggers tend to be surprised at how many such buckles were on a wagon-pulling horse's harness. See the drawing below.
Your first item, the 2"-long rectangular, may not be a buckle. Wagon-pulling horse body-harnesses usually had a "frame" like that as a ring-like connector where three (or four) straps came together. You'll see several places in the drawing where that occurs.
We relic-diggers (which includes me) tend to be surprised when we are told our rusty iron horse-harness buckle is STATISTICALLY unlikely to be from the civil war... because CIVILIANS used MULTI-MILLIONS of horses for pulling wagons, from the Colonial Era all the way into the 1930s. (Photos of American cities in the 1930s show dozens of horse-drawn wagons still being used in the streets.)