What you want is called "Fluid Bonding."
Here is one short article about fluid bonding aka "condom contract" and poly: Safer Sex Options
Here is another interesting short article about it (substitute the author's term "two people" with however many people will be bonded with you): Fluid Bonding
Here's another discussion thread on this site about fluid bonding: fluid bonding/bareback
You may also be interested in this thread: Safe Sex - Standards, Practices, Information & Resources