Isometric drawing in AutoCAD, complete tutorial 

You can use FLATSHOT command for this, more details in this video ru-vid.com?o=U&video_id=PlS5rgVKjz8

You see the 3 lines at the end of the status bar? When you click on it, it'll give you several coordinates. Click on isometric draft to add it on your status bar.

i am not able to understand question properly Rajeev but as far as I can figure out you are asking about joining multiple geometries end to end on different planes. If that is the case then you can change plane each time you make a new geometry and it can be joined end to end to form a closed loop.

Imagine a rectangle in which 3sides are drawn by straight lines and the these 3lines are joined by a spline which is in perpendicular plane to the 3straight lines. My question is how to join these spline and 3straight lines and how to make them become a surface.???

My apologies for late response Rajeev, you can make use of NETWORKSRF command for making complex surfaces but it also has limitations as it cant make a surface which is too complex as in your case edges have a an angle of 90 degrees so it might be difficult for AutoCAD to handle.

Yes you can do these drawings in AutoCAD 2012, for changing your plane to isometric type DS on command line and press enter a window will appear. from this window select "snap and grid" tab and from snap type panel change it to isometric snap. then rest of the instructions are same as mentioned in the video. see this image for more goo.gl/cXsZtb

Yes you can here is a video to help you with that ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-orIGydXXvVo.html