1) signal 3 sec ahead
2) checkback is before u move off. Because u need to ensure that u can move off safely.
While blind spot is while u want to turn or filter into other lane. The result for blind spot is because there are area where your mirror cannot show, so those area that are not shown in your mirror is blind spot.
http://www.google.com.sg/imgres?imgurl=http://www.motorcyclebasics.com/images/content/blind21.jpg&imgrefurl=http://www.motorcyclebasics.com/blind-spots.html&h=500&w=364&tbnid=SjuPngWs1qkXoM:&docid=RMiCLc6exubPnM&hl=en-US&ei=2UAeVrbOEsWeugTI7