We cannot get the value of A3 question in A4 question using SSI_GetValue instead use perl function to retrieve the value of A3.
We can use SSI_GetValue function to retrieve the value of a variable which is present in page currently displayed. For getting the value of already answered question, we need to use perl.
I have edited your piece of code and placed below,
(SSI_GetValue("A4_r1_c1") > [% A3_r1_c1 %] ) ||
(SSI_GetValue("A4_r2_c1") > [% A3_r2_c1 %] ) ||
(SSI_GetValue("A4_r3_c1") > [% A3_r3_c1 %] )
strErrorMessage = "Error in A4.";