Cost per rig or percentages

Hi all good morning.
I want to add more rigs to my farm on HIVEos, something is not clear for me.
I have now 1 rig with 6 GPU, i would like to add 4 more rigs with 6 GPU each and 2 rigs with 12 GPU each. In total is going to be 7 rigs with a 54 GPUs.
I saw i can choose to pay 3% of earnings or 3$.
Maybe is not clear only for me but if i choose to pay the fix price, 3$/month, It is per rig or per GPU?
Am i going to pay 21$/month (7 rigs x 3$) or 162$/month (54 GPU x 3$) ?
Many tks

It`s not clear to me too. Also, if we choose to go with 3% pool fee instead seems to loose alot more money then 3$ per rig, no?

Yes exactly, i mean if i have a 13 GPUs rig, for witch reason i should choose to pay 3% of the revenue…