Could anybody tell the equation of 3-element control loop program in ML1500, or equation of the loop.

i am going to commission the boiler, so this is conflicting in my mind

The ML1500 PID equation is listed in the instruction set reference manual. You can download the instruction set reference manual from AB's website, http://www.ab.com. Click literature library on the left hand side of your screen and navigate to the MicroLogix 1500 PLC manuals.

Could anybody send the program of 3-element PID. there is a problem on site and pid loop is not working properly.

just for reference..

Is it possible that you may be confusing 3 element Drum Level control with the fact that there are 3 parameters in PID. You need to understand the concept before evaluating if the program is correct or not.

There is an explanation of it here:
http://www.controlguru.com/wp/p44.html

There is also document on the Siemens USA site that explains the basics of 3 element control here:

http://www.sea.siemens.com/instrbu/docs/pdf/ad353-105r2.pdf

I hope the information helps.