yes
in image 3, the width of the gang is 50, and this will put all the bore center among x axis, just like a stacked internal toolholder
if you mount, inside each one:
... a small boring bar / picco, then you will zero each one just like you said
... a drill, or whatever similar tool, then you will need to ensure that it is coaxial with the spindle, and only after that proceed to offset declaration
however, before measuring the tools, it is ok to check the gang itself, empty, without tools in it; this is because normal alignment procedure is targeting coaxiality inside the bore of an internal toolholder, that has it's axis at X0, while the gang may have bores at X<>0; it may not seem important, unless you use small tools; as you increase tool size, then also they will be able to handle more missalignment; in reality, there is a big chance that the gang won't be aligned, even if your turret alingment is ok; maybe, at this moment, it is too soon to talk about this, but just be carefull, because skiping this step may break some tools
or maybe it won't happen
considering that gang is at post 1, then you may use offsets :
... 1 13 and 14, thus inside the program will be needed T010101 T130113 and T140114
...... or
... 1 21 and 18 : T010101 T210121 and T180118
some persons preffer to always use the 2nd offset as being 1st offset + 20; this is kind of a simple rule, so to keep offsets number organized, but it works only for 2 edges / turret post ( like only dual gang toolholders ), and only for simple lathes; it may seem to be a limitation, but truth is that most lathes are simple, and there are rare cases when is neded to use more than 2 edges / post
for example, there are rare cases when someone uses a gang, just like you wish