Can anyone please tell me the exact purpose of 3 resistors connected with MCS2 contact?
I think those are field weakening resistors not braking resistors. 

When the start button is hit the field is initially supplied directly from the line via MCS1, then once thing thing has gotten to a moderate speed the timer TM1 switches on MCS2 which once it pulls in drops MCS1 via an aux contact. 

The effect is that 10 seconds after hitting the start button the field current is reduced causing the shaft speed to increase. It is a measure to reduce the starting current (And limit starting torque).