Wow. You say "contributing" meaning you are already in this deal. A deal with no agreement made in advance.
Is this an investment property or do the three of you live there?
If you live there - you each are in for one third, but first, there were 3 deposits, so the excess the two higher deposits put in gets to them first. And the monthly extra as well. For example:
A put in $5000 more than C, plus $500 more each month. After 60 months, it's $35000 more. So on sale, A gets $35,000 B gets $14,000, and then it's split 3 ways.
There are probably a dozen ways to divide things, for example, I ignored interest, does A also get 4%/yr added to the running balance? Are the rooms all the same?
The other way is to divide the property shares based on the deposits, so you each own an amount different than 1/3. How far away from 1/3 are the deposits?