How should I structure a finder's fee that has multiple referrers?

Business A called me to ask if I knew a competent PR firm to handle a large project for them. I asked Colleague A, who referred me to Colleague B, who referred me to PR Firm 1. PR Firm 1 gets the job and offers me a finder's fee of 5%. I feel obligated to Colleagues A and B to cut them in, because without their referrals I never would have found PR Firm 1. What kind of percentage should they receive, if any? Yes, I will be needing a contract.