When shifting to the 1st gear the chain shifts up to 3rd derailleur(rear)! I am using 21 speed bicycle and the problem is that I am unable to shift the chain to the largest derailleur(rear)....the shifters are thumb press type there's no problem in pressing the shifter...when shifting to the 1st gear the chain shifts up to 3rd derailleur(rear)!
A stretched cable is one possibility. Has the cable been replaced recently? They stretch most when they're new. 

A second possibility is that you removed the rear wheel and when you put it back on, didn't get it seated the same as it was before you took it off. That could change the position of the derailleur enough to make it impossible to shift onto the largest or smallest gear in the cluster. 

A third possibility is that you need to adjust the high-low limit screws on the derailleur. Here's what Park Tool has to say about that... http://www.parktool.com/blog/repair-help/rear-derailleur-adjustment