I am using a term reference field but the widget type is autocomplete term widget. This term consist of many levels as parents and child. In the form I need to display only the child terms(only the lowest level in my case 3rd level) in this field. How can I do this. Please anyone help me on this. Thanks
There's a GREAT module for this type of configurations called Hierarchical Select
Hierarchical Select has the ability to save the entire lineage of a selection or only the "deepest" selection. You can configure it to force the user to make a selection as deep as possible in the tree, or allow the user to select an item anywhere in the tree. Levels can be labeled, you can configure limit the number of items that can be selected, configure a title for the dropbox, choose a site-wide animation delay, and so on. You can even create new items and levels through Hierarchical Select!
This module has a lot to offer, but as for your need, it also does the trick. Also you might want to consider Taxonomy Term Reference Tree Widget as well, the main functionality is a different thing but you can define a permission via views to do what you are after too with this module.
Maybe the Hierarchical Term Reference Autocomplete module could help you. It seems to combine the restriction of level of Hierarchical Select module (select only level 3 for you) and the Autocomplete widget.