It sounds like you've miss understood the sarve tool a bit. It also sounds like you've done some Unreal mapping since that's how the "making 2 basic rooms and connecting them with a hallway" type tutorial would go (Since you would simple make 3 subtracted brushes)... Anyway, the carve tool is used to carves gaps in a brush (you're actually splitting the brush into several). You use a brush to carve into the other brush. So making a hallway between two rooms the way you're trying to do it would require you to also carve out of the middle (hallway) brush so that you can walk through it. - That's all a really bad way to map using Hammer so forget it.
It's better if you start mapping with Hammer forgetting about tools such as carve and construct everything by making each brush from scratch. Make a floor, walls, ceiling - 6 brushes. Then continue to split up your wall manually to make the gap where the hallway will go - 4 brushes. After making two rooms like that you would do the same again for the hall except all you need is the floor, 2 walls, and ceiling - 4 brushes. There's your "two rooms connected by a hallway" for Hammer.