Three people times three months is nine, and a man-month costs at least US$10,000 in the US. So $90,000
A man month of a real professional's time at a real startup costs $10k.
At a no-money/no-office indie startup, with inexperienced staff (who have no serious financial commitments, and an underdeveloped sense of self-worth combined with youthful enthusiasm and misplaced passion), a man-month might be closer to minimum wage (~$1250) with no additional overheads.
I know this is an outlier as I've seen hundreds of these projects completely fail... but I know of one studio who managed to get by on ~$40k/mo, which went to half a dozen directors' salaries plus office overheads, while paying a dozen other staff in nothing but promises of profit share upon release! I mention this not because it's a sensible plan that should actually work... but just that I do know of one case where it magically, somehow did work...
I see... so barring spending tens/hundreds of thousands on marketing, is there anything I can do?
Slowly, over many years, build a reputation of consistently making great games so that Apple/Google features your game on the front of the App Store.
Or make games as cheaply as possible so failing to repay costs is less of a risk, and churn out an endless stream of crap games, hoping that one will be a viral hit.
Or do it as an expensive hobby instead of a badly run business.
How much would you say does the average game make...
As above, the typical (median) game probably makes nothing. There's many, many millions of games out there, and most probably haven't even been downloaded. A handful of mega hits make hundreds of millions of dollars though, so while the median game probably makes $0, a meaningless average of a hundred million bad games + a hundred mega hits = (100000000 * $0 + 100 * $100000000)/100000100 = the average game makes $99.99 
That just goes to show that averages aren't very useful though!
