Assuming your ownership structure is straightforward--each owner's % ownership correlates directly to their share of profits--the general formula is (Individual Contribution) = (Total Partner Funds) * (% Ownership in company). Here's a basic example, you can plug in your own numbers:
Total Capital Contribution Required Currently: $50,000
Total contributed == 15,000 + 15,000 + 20,000 = $50,000.
There is plenty of room to complicate this simple breakdown, but that's the general one that I would start from.