SQL member agreement across tables

Ok, excuse the verbose question, but I’m new to MySQL and PHP, and my brain is hurting trying to work through this one in my mind:

I have 3 tables that look something like this:

Id Membername
Var1
Var2
Var3

Activ Membername
Activ1
Activ2
Activ3

Dat Membername
Dat1
Dat2
Dat3

And I want to create a select which will pull an individual members information across tables (say membername=1) and compare that member to all other members in the database (say membernames = 2 through 9) resulting in an output that would look something like this:

Agreement Membername
100% 4
97% 5
96% 2
94% 6
90% 7
88% 9
70% 3
50% 8

Essentially it’s an agreement measure across all fields and all members of the database that needs to be able to be run one member at a time.

Someone told me that I CAN’T use a subselect and since then my head’s been spinning.

Any help would be appreciated!!!