We use the recursive definition step-by-step:
Step 1: Start with $f(1,2)$
From the 3rd rule:
\[
f(1,2) = f(0, f(1,1))
\]
Step 2: Evaluate $f(1,1)$
Again from the 3rd rule:
\[
f(1,1) = f(0, f(1,0))
\]
Step 3: Evaluate $f(1,0)$
From the 2nd rule:
\[
f(1,0) = f(0,1)
\]
Step 4: Evaluate $f(0,1)$
From the 1st rule:
\[
f(0,1) = 1 + 1 = 2
\]
Step 5: Substitute back
\[
f(1,0) = 2
\]
\[
f(1,1) = f(0, 2) = 2 + 1 = 3
\]
\[
f(1,2) = f(0, 3) = 3 + 1 = 4
\]
\[
\boxed{4}
\]