Not such a 'noob question'; I have to look up how to do this every other day!
You want the function Sequence; note that it takes a list of arguments, not a single list argument. The @@ below uses the elements of replacement as successive arguments for Sequence.
list = {{1, 2}, {1/2, 1/3}, {3, 4}};
replacement = {{4, 5}, {6, 7}, {1, 2, 3}};
ReplacePart[list, 3 -> Sequence @@ replacement]
