I think the issues you'll be dealing with are something like this:

1. BCD- that's the Bolt Circle Diameter- basically, the size and pattern of how the chainrings connect to the crank. Most ATB/MTBs have a 110 for the outer ring and 74 for the inner ring. A lot of "road" cranks are 130. Older ones are 144, there are some other gooney ones as well.




2. Bottom Bracket Spindle length. Generally, the spindle for a triple crankset will be longer than one for a double. Makes sense, right? Three rings take up more real estate than 2. If you switch from a double to a triple or vice versa, you'll generally need to replace the spindle or BB unit.

3. Chain length/Derailleurs. Your triple setup is going to have a chain that's long enough for your current chainrings. If you get bigger chainrings- you'll probably need a longer chain. If you get bigger chain rings, you may need to move your front derailleur to clear the chainrings. You might want a shorter cage rear derailleur because you won't need that extra cage length. Or you can keep it because it looks cool.

Your secret plan about the trekking bars is a good one. What makes it even better is that your components will fit on that bar... I mean the bar diameter is the same from a flat bar to a trekking bar- but road drop bars are different. I'm still working on getting my trekking bar set up to my satisfaction. I may actually start looking for a different style of trekking bar- and there's a huge range of them, different bends, different sizes and stuff... And I really want to clean up the cable routing...