Apparently I'm fucking brain dead because i can't come up with a formula nor logic for a spreadsheet

Person1 and Person2 start a fund with $50 each, making it a total of $100. Which gives them 50% each of current holdings and future profits.

After a while, the fund starts to profit and the total amount grows to $200.
Person 1&2 still own 50% each.

Person 3 joins the fund with $50.
The fund now has $250 to invest and make profit.
How much % of the fund and future profits would Person3 own?

Can someone help me with how to logically process this so a formula can be done?
I'm totally blank on how should I operate on this.

I'd like to input the current funds and depending on whatever person1, 2 and 3 has invested get the percentage of it.
Should be applicable to person 4,5 and so on too?