Actually all of you contributed to the solution! Thanks a bunch, here is the new code:
_f1=getpos ftrs select 0
_f2=getpos ftrs select 1
_f3=getpos ftrs select 2
#start
player sidechat format ["fortress location is %1,%2 and %3",_f1,_f2,_f3]
~1
_aaa=(_f1) - (getpos player select 0)
_bbb=(getpos player select 1) - (_f2)
_ccc=((getpos player select 2) - (getpos ftrs select 2))
player globalchat format ["a is %1, b is %2, c is %3",abs _aaa,abs _bbb,abs _ccc]
~1
goto "start"
I think the main key was naming fortress1, since it returned scalar bool array blah blah blah without a name.
I would have liked to use the distance command, but as I understand it, it would give me the slant range of 2 objects at different heights and locations and what I really needed to build this script for is to get the ALTITUDE ONLY (select2) difference. Now I can continue with the rest of the script, thanks to everyone for your help ;D