Thank you all for giving replies and sorting out my problem the problem with uln2803 was not the 3KHz frequency being so high,
1. The actual problem with my circuit was that I had not used pull up resistors at all across my uln2803 outputs after putting a 1K pull up this problem has disappeared.
2. As far as the response of stepper at 3KHz is concerned my drive is a half stepping one so for every pulse sent it rotates my motor through a half step i.e. 0.9 degrees. thus 400 pulses makes one revolution of motor. 3KHz pulse thus rotates my motor at 450 RPM this speed is not too high for sanyo denki bipolar 50Kg-Cm motors which I am using obviosly I am strating my motor at 60RPM and than accelarataing it gradually to atain 450RPM.
3. Some of you guys had asked why I am using ULN2803 even optocoupler outputs are sufficient to drive the motor drives. The reason for using ULN2803 is I have to control 6 stepper motors i.e. 6 drives through P0 of my micro. Thus I have 6 "start" signals for these 6 drives to select a particualr drive and the "CLOCK" and "DIRECTION" signals I have cascaded i.e. same signals goes to all the drives. and when the pulses on clock signal are given only those motors will respond whose start signals are enabled. Since clock and direction signals are cascaded thus my driver has to sink current from 6 devices i.e. 6 motor drives on these signal lines. I was afraid if my optocoupler would be able to sink current from all 6 at a time. So to avoid any problems which could latter arise I used ULN2803.

Thanks to all the people who responded and gave their valuable comments and suggestions.
Bye,
Prahlad