The tan and white is either part beagle or possibly basset hound. If the black dog is from the same litter, it would indicate that a black lab was somewhere in the blood line.
Often when trying to determine the origin of mixed breed dogs, you really have to go back tow or three generations in many cases. I own three beagles and can attest they are not full breed beagles, but they are cute pups. I suspect they would grow to be large than the average beagle, which is why I suspect a basset hound heritage may be present.