Hi sivakumar,
MSG3 buffers is sent on SRB0 in initial time. UE can do RACH either to get sync or initial access. If it is a initial access or attach ,UE will send RRC connection request in msg3. If it is a synch RACH then UE will send MAC_CRNTI CE to delete the new RACH rnti. One more synch loss UE will send RRC re-establishment message in msg3.
First synch loss is either TA timer expires or SR count reached MAX.
second synch loss is while in data pumping if RLC retx reached MAX ,etc .
