Du kan gøre det med en GROUP BY:
select CLUSTER_K,
max(case when PROCESS_PHASE=1 then SEM_AGG else null end) SEM_AGG_1,
max(case when PROCESS_PHASE=2 then SEM_AGG else null end) SEM_AGG_2,
max(case when PROCESS_PHASE=3 then SEM_AGG else null end) SEM_AGG_3
from test_mc
group by CLUSTER_K