I am assuming you have Q3 on a separate page from Q1 and Q2.
You can skip Q3 if Q1 is only answered once. Use the SSI Script function NumChecked(Q1)=1 at Q3 to handle the display of Q3,
You can then use the SetValue function and some Perl script to set the correct Q3 item to the value of Q2 if Q1 is only answered once.
Insert this Perl script at the footer of a question after Q3 ...
[%Begin Unverified Perl
for($i=1; $i<=7; $i++)
Run some test surveys and check your data via the admin module. I got it working just fine.
Note 1 - you may consider making Q3 a constant sum question if you want all items at Q3 to add to the Q2 value. You can insert [%Q2%] as the total value.