What do you want to know specifically:
What percentage is each person entitled to?
(((((800,000-150,000)/3)+70,000)/800,000)*100) = 35.833333333% - for the $70k person
(((((800,000-150,000)/3)+40,000)/800,000)*100) = 32.083333333% - for the two $40k people
It would be much easier for each person to just do a $40k down payment unless you're trying to avoid PMI or something but $150k of $800k is 18.75% so you would still need PMI.
How much does each person need to pay monthly to have 1/3 ownership evenly?
This is tough because the person with higher ownership would need to be put on a sliding scale to go from their rightful 35.8% ownership down to 33.33% as time progresses.
You really need a good lawyer and accountant to figure out the contract and figures. Napkin math is only gonna lead to guaranteed disputes. Don't forget that you will need to equally account for repairs and renovations.