Yea from what you've described they're definitely avoiding you for some reason. Co-worker A seems to be toxic. Or He may also be austic or just very socially awkward. And yea maybe consider going to HR. For me, personally, I'd give them a chance to explain themselves and I'd be curious to find out that's going on in their heads. Maybe you can sort things out with them. And tell them that what they're doing is not okay and that it's bothering you. Explain that you simply have a speech impediment and you're starting to feel ostracized. If they still don't make an effort to create a friendly work environment then go to HR and explain what's been going on.