I have heard that in Islam it is not allowed to mourn over a material loss (such as a business loss), and for a non-material one one is not allowed to mourn for more than three days.
Can someone share some ahadith/ayat about this?
jazakallah
Re: Mourning
I do not know of any hadith/ayat, I am sure alot of the other brothers here do, but I think its great. I mean mourning over a financial loss only makes you weaker, and will ensure you never bounce back and stand up on your own feet. Once again, this proves that everything in Islam is logical.
Here is something to always remember:
2:153 O ye who believe! seek help with patient perseverance and prayer; for Allah is with those who patiently persevere.
2:154 And say not of those who are slain in the way of Allah. “They are dead.” Nay, they are living, though ye perceive (it) not.
2:155 Be sure we shall test you with something of fear and hunger, some loss in goods or lives or the fruits (of your toil), but give glad tidings to those who patiently persevere,
2:156 Who say, when afflicted with calamity: “To Allah We belong, and to Him is our return”:-
2:157 They are those on whom (Descend) blessings from Allah, and Mercy, and they are the ones that receive guidance.
^^
jazakallahukullakhairun sister/brother 
and your signature is surely interesting 