That's as thin as saying Pale Rider, Two Mules and Unforgiven are a trilogy because he wore a cowboy hat.
Ha. Wearing EXACTLY the same cowboy hat, poncho, etc. at various points AND intentionally through three movies wasn't an accident. Again, I don't like the idea but it's absolutely intentional.. even if it's a nod.
Suggesting Manco could be Blondie is as nonsensical as the idea Col. Mortimer could have done a heel turn and become Angel Eyes. It doesn't even pass the squint test.
Well, a few problems with that:
1. It's not nonsensical. If you're going to make this argument, at least get the chronology correct. GBU is unquestionably earliest due to the CW involvement... so your opinion is already flawed and misinformed. (P.S. Since AE is dead, it still works.)
2. Reuse of actors or repurposing is hardly a leg to stand on. Are you going to tell me that Hateful Eight, Pulp Fiction, True Romance, Natural Born Killers, Inglorious Basterds, and Reservoir Dogs are all in different universes and totally unrelated because of actor reuse? Or would you tell me they're at least related because of items, characters, and objects are intentionally inserted and noted on screen? (Pack of red apples anyone? Scagnetti? Lee Donowitz?)
There are conscious, intentional ties between the movies. Whether they're "in the same universe" is up for debate* but there's nothing you will cite that will absolutely exclude the possibility.
*I don't know why people need anything to have a "universe" or "canon", it's a stupid concept for people who need things to be anchored to like them.