Fastest way to disable indices and drop constraints and rebuild SQL Server table
If you just want to Truncate and reload data, then faffing around with indexes is not necessarily useful. 

If you're inserting data in Clustered Index order i.e. in CONCEPT_CD ASC order, then there's no real advantage to dropping the Clustered Index. It'll be far more pain rebuilding it at the end on 3 billion rows than it would inserting the data in your Clustered Index order in the first place.

However, if you want to disable your indexes, then that would be as follows:

-- Disable indexes on OBSERVATION_FACT
-- If you're dropping, don't disable. If you're disabling, don't drop...
ALTER INDEX ALL ON OBS_FACT DISABLE;

-- Truncate your table    
TRUNCATE TABLE dbo.OBS_FACT;

-- ETL Process here....

-- REBUILD indexes on OBSERVATION_FACT
-- Or recreate them if you've dropped them in step 1
ALTER INDEX ALL ON OBS_FACT REBUILD WITH (ONLINE = ON);


Creating following by immediately rebuilding your indexes as per your script, is pointless, as creating builds the index. Why then immediately rebuild?