If you are not a veteran stick to TWO fleet types, not three. Either give up on the small short range turboprops or give up on the long range long haul jets (or give up on medium/large planes in-between).
This is what Sami wants you to do, this is what the veterans want you, new player, to do.
Only veterans can operate three or four fleet types thanks to their inter-alliance plane trading and cheat sheets (plane performance and cost accounting charts, history of best routes from all major airports, entire game length pre-plan of fleet choices, etc.).
The only thing I don't understand is why we have to figure this out the worst way: by adding too many fleet types and going bankrupt.
Just write somewhere, clearly: you can only have two fleet types so that when the time comes to swap them you won't go bankrupt. You may see some airlines with more than two fleet types, these are the superior players who will replace hundreds of planes all at once within 24 real life hours thanks to their alliance helping supply planes. You are not them. You cannot do this. Accept the fact the rules are stacked against you and in their favor.