You can set three different pools. The miner will always run on the first pool and the other two would be backups I case the first pool dropped out. You should see a setting on the configuration page saying "failover" you can change that so it balances the work across al three pools or set up a quota so it does the same amount of work on each etc.

You will get more errors if you share the miners work across all three and obviously you will see less of a payout from each pool as the hashrate will be shared. So 6Th/3 would be approx 2th per pool.