The key there is to be honest :yup: Let them know you are seeing others.
Trust me, they will want to see more of you because they want to "win" your affections...
Seriously, honesty is the best policy. Plus, you're not playing games that way and you get a chance to see who these girls really are. Who wants to be around for the right reasons and who doesn't.
Go out with them all! Just don't mislead them.