I am new to RC field and am trying to construct a tricopter..
I need to know how to run my brushless motor? I need a perfect motor driver and so can anyone send me the circuit diagram?
Apart from using ESCs are there any methods to run a BLDC motor?
Thanks in advance..

The ESC is the only way to run BLDC motor for flying a multicopter, If you don't use an ESC you can't alter speed of BLDC motors.
ESC is also a micro controller based electronic circuit. the ESC's usually have 8 or 16 bit micro controller, voltage regulators,high power MOSFETs for giving sufficient output power to each poles of BLDC motor, the MOSFETs will be controlled by the on board micro controller according to i/p ppm signals.

I would not recommend designing your own ESC for multicopter applications, because main problem will be weight, you may have to use SMD components in a very tight board design to overcome this problem, but i am not discouraging you if you have access to dual side smd pcb manufacturing and smd wave soldering,at cheap price go ahead, it will be of great help if we could produce RC accessories in our own country, If you are  really sticking to this idea, you can use open source ESC firmware like simonK for you build.
 I am giving some useful links, struck anywhere?, feel free to contact me, eager to know your progress  

http://www.rcgroups.com/forums/showthread.php?t=1908901     (ESC circuit with simonk firmware on Atmega8)

http://aeroquad.com/showthread.php?6741-My-DIY-ESC-design-can-somebody-check-it-over

I have a doubt. How are the bldc motors controlled. Which method do they follow: Sensored or sensorless control?

In a multirotor, motor control is sensor less, the sensors in mulrtirotor does not sense the motor speed but current roll, pitch and yaw(sensors does not provide direct roll , yaw or pitch o/p the flight controller calculate this values from the sensor output.) and the bldc motor speed is controlled according to these values (which all are done by FC board ofcourse) The ESC only takes the pwm motor control output from the FC and controls the motor according to it.   

So Hall sensors are not used. Am I right?

No never in a multicopter
In multicolor not the speed of each motor but the Euler angles of the whole system is sensed and motor speed is controlled according to these measurements.  

Got it thanks.

So don't forget to upgrade your progress, 

One more quick suggestion, why you chose tricopter for beginning? is there any special reasons? if not, i would recommend a quad for a beginner.
Though, It's completely up to you , what you make !!

Quadcopter seems a little expensive. So we decided to go for a tricopter.

May the crashes costs more  , anyway the choice is yours 

I thought that, But the more price you say is only one ESC+ Motor. Tricopter may cause you headaches.
Is this Build part of any project? or just hobby?
What is your applications with this Tricopter?
What is your current configuration?(motors,ESC,Prop etc..)

And finaly I can provide some Really good deals for you, For Eg, 4x A2212 1000Kv motors for 2000 only(new ebay).

PM me for details or call 9656000639

You said that right akhilzid !!