3 seconds is the default value for alc’tel network, you should also check that the T_IA is lower than T3101 (T_IA = 2.5s default)
So if your parameters are correct, you have to find the cause for the failure : SDCCH congestion ? assignment failure due to radio ? assignment failure due to BSS ?
In alc systems, the normal sdcch assignment failure rate is about 10% to 20% in rural areas, and about 5% to 10% in urban areas.