Differences between: 过来, 起来, 下来
过来 means "come over/ come here", 坐过来 means "sit over here"

下来 means "come down/ get down here) 坐下来 means "sit down"

起来 if used by itself, means "get up/ stand up", also carries a sense of "direction." when used in a combo like 
说起来 (when being talked about) 做起来 (when being worked on) it carries a meaning of "being brought up" or "being initiated." In this case, its usage is not necessarily indicative of "direction."

all three can be used by themselves or in a combo like the examples i give.