With 'Three Man Army', I also found there are times you won't be able to do it. When I went for it, I was playing as Michael and just talked to Franklin, and when I called up Trevor to hang and then Franklin, the latter stated that we should focus on something else right now. Perhaps you have to wait until after all three meet as one? I'm not sure.
Also, with 'Trading Pure Alpha,' if you don't want to risk your money, there's a random event where a man's car has broken down and he needs you to rush him to the airport. When you get there, he will tell you to buy certain stocks because you're guaranteed to make money. It seems guaranteed that this will 100% net you some cash, though it isn't a big risk otherwise. Hope this helps anyone having troubles.