I don't know if you mean the 18x20 3-LNB dish, or a new slimline 3 LNB dish, but neither will receive Greek programming. Those channels from D* are on 95W, and you need a different dish with a Ku FSS LNB.
You don't necessarily have to buy from Solid Signal, but this is the dish you need.
If it's at all possible (and I don't know your situation), call DirecTV and request the programming and they will schedule an install of this dish. I have not installed one personally, but everyone seems to agree they are not an easy dish to put in for the average person. But that's up to you.