Sorry for my late reply.
From what I found out, T3121 only applies to 2G to 3G handovers, so I don’t think tuning it will improve your handover success rate in any way. In any case, 20s is long enough for a handover to take place.
T3103 is used to reserve the old channel so that in case the activation of the new channel fails, the call can return to the old channel. Thus, tuning it will also not improve handover success rate.
T8 applies in inter-BSC handover and T7 in inter-MSC handover. Unless your problem is related to these, you should not touch these timers.
From my experience, the majority of handover failures occur as a result of interference, hardware faults or poor synchronization. If your target cell is congested, you can also have handover failures. I think you should focus on these, rather than the timers.