The bank will never "call in a loan" by selling off secured assets (at least not so far for me). They'll just close your airline.
I think I was in the red for about a week (real time) before I got a first bankruptcy warning, but it probably depends on how indebted you are, and how fast you're burning cash. After that you get six days (again, real time) to improve the situation.
My airline just got closed by the bank. I started fairly aggressively with LH flights with leased DC-8s. It was all going gangbusters until the C-Checks started rolling in - which is what killed me in the end, as I had to store them. It's hard to pay for 30 stored, leased aircraft with 18 DH-114s!
As for having one of each of three fleet types, that will never work. As noted, the commonality costs will kill you.. it was hard enough with two dozen DC-8s.. when you don't have cash to spare on the cost of servicing older aircraft (the time taken to get a decent RI for these planes was probably the biggest killer in the end.) Even leased, you would save on storing them, as there would be no commonality or servicing costs for them, regardless of how much money they're "making".