So, I guess this means you’re using your action bar numbers 1,2,3,4 etc. that change when you swap stances.
I’m curious if you’ve copied the macro correctly or what’s going on with it. It seems to show:
line 1
#showtooltip [stance:3] Overpower; Whirlwind
This reads to me like if in stance 3 (which is berserker stance) show the Overpower tooltip, otherwise show the Whirlwind tooltip.
line 2
/cast [stance:3] Battle Stance; [stance:1] Berserker Stance
This says if in stance 3 (berserker stance) cast Battle Stance. If in stance 1 (battle stance) cast Berserker Stance.
Line 3
/cast [stance:1] Overpower; [stance:3] Whirlwind
This say if in stance 1 (battle stance) cast Overpower. If in stance 3 (berserker stance) cast Whirlwind.
To me, only the last line makes sense, and it’s not doing anything if you are in Defensive Stance, which I suppose is fine if it’s only on your bars for stance 1 and 3.
As a sort of soft introduction to Classic Warrior, and macro building, I’d recommend having a look at Elite’s Warrior Tanking Guide, from way back in September 2019. It’s not 100% relevant to SoD, but it’s not far off. There were no runes in WoW Classic 2019.