Hi there Haider,
Since I got the same questions in our support ticket # 00793811, I'm pasting my answer here as well:
Q1. It is not recommended to make your Core a PXE Representative:
http://help.landesk.com/Topic/Index/ENU/LDMS/9.5/Content/Windows/osd_c_using_pxe_representatives.htm
Q2. You can as the PXE representative is able to act as a server for multiple client at a time.
Q3. Our recommendation is to use a PXE Representative for each subnet where you have target devices to provision. This is the simplest scenario. However, it is possible to use single PXE Rep for multiple subnets with a proper configuration of DHCP Relay Agents and DHCP option, as explained in this community article:
How to use a PXE Representative for multiple subnets
https://community.landesk.com/support/docs/DOC-23909
Hope this helps
Francesco