Wonder Woman should be able to walk all over them pretty easily. She could take out Bats and Cap almost immediately and then deal with Spider-Man with perhaps minor difficulty. I seriously doubt though that she's the weakest character that could take all of them though. She's pretty damn good. It's just that she's the weakest I can think of right now. I've been up most of the night and all day today so I'm drawing a blank right now...
What about Iron Man? I'm not sure if he could take all three but he'd probably make a good fight out of it. I remember him tangling with Spidey many years back when Peter's secret identity was revealed and Tony seemed to be struggling against him, iirc. So I kind of doubt he could take Spidey if Bats and Cap were helping out.