Amateur question here (question from an amateur [excuse me if I have the lingo wrong]...):

I have a 11-speed Shimano 105 rear derailleur on a Giant Fastroad Comax 1. The front derailleur is also a Shimano 105, crankset is a Shimano RS500, with 34/50 chainrings.

I also have an older bike, 21 speeds in a 3x7 arrangement. And when I use the older bike's granny 3rd gear (1st gear? I'm talking about the "lowest" gear) in the front combined with the lowest gear in the rear, it gets me up a steep hill with less puffing than my newer and much lighter bike on its lowest combination of gears. It's not a huge difference, but my heart and lungs can tell...

Can I swap out the 34/50 setup up front to something with 3 sprockets so that I can have a lower granny gear like I do with my older bike? If this can be done, will the left shifter (Shimano RS700) be able to work the new 3-sprocket setup properly?