I bought a house with the pipes cut at the slab. There are 3 pipes coming out of the slab, one is the main line 3/4 inch copper, then there is another 3/4 copper and a 1/2 in copper . I think the two 3/4 copper go to the hotwater tank. Main to the blue cold water side and the other to the hot water side. My question is what is the 1/2 line for?

Depending on the lay-out of your house and location of these cut-off pipes there are few ways they could be connected. You have to test it to determine which one is which.

However, If you think one 3/4" pipe is main cold going to the heater and the other is hot then connect them and leave 1/2" pipe cut off. Turn water ON and you will see which plumbing fixture doesn't have hot/cold water running. Then, connect it to the right pipe.

For testing purposes, buy Shark Bite fittings, piece of PEX pipe and start connecting them together until you get it right. PEX pipe and Shark Bite fittings are easy to install and deinstall.

Hope that helps

Milo

I'd like you more details. Do you have water pressure in the house at the present time? Is this new construct? Are there any PVC pipes coming out of the slab?
Back to you, Tom