Timer t3101 is the time the network waits after sending the IMMEDIATE ASSIGNMENT or IMMEDIATE ASSIGNMENT EXTENDED message until the main signalling link is established before releasing the newly allocated channels. The value for this Timer can be between 50ms – 1sec. It is vendor dependent. The current value of this in ur network is 3sec, that is quite high but it will not effect ur networks Low Immediate Assignment Success Rate but it may increase the call set up time in the network.
To increase the Immediate Assignment Seccess Rate firstly find the bad cells and check whether whole site or single single cell in the sites have problem. If the problem is in whole site then it regards to hardware failure or points to Media. Else for other thngs check ur Frequency Plan Co BCCH Co BSIC conflicts, the Max TA from which ur cell is taking the call, Uplink and Downlick Interference and Link Budget.
Hopefully this will help u out.