and really, you don't HAVE to be shady about this. seriously.
Hi, 20 year old?
"Baby, you can do whatever you'd like, and I'll do whatever I'd like. I'd love to hang out with you every Wed and Sat nights, hitting the town and the beach and having a blast and having mind blowing sex. We can text and chat during the week, but I'm pretty busy on the other days. I'll never ask you to limit your social life - hell, life is all about just getting as many enjoyable experiences as possible, right? See you wednesday... wear that lacy red thing."
Then you have that conversation with the 40 year old, limiting your fun to a 2 hour romp every sunday night to get the week started right.
Then you spend as much time with the 30 year old, until you decide she's the one you marry. Or you don't decide that, in which case you keep things like you have it with the 20 year old. Just be careful - if she's not a single mom, she IS gonna want babies.
Oh, and one more thing - get a damn vasectomy before you try any of this. Tell the 40 year old. Dont' tell the rest. Oh, and date the 30 year old where you live, and the other 2 from adjoining counties/towns. You'll be glad when you're not running into the fast ass young chick with her crew at McD or the older chick with her 'separated' hubby at the Sears.