First and foremost, that would require Q1 and Q2 to be on separate pages. I would actually ask Q1, then page 2 with Q2 you would have a custom script that would check this first question in the manner you described and if >=3, it would jump to page 3, they would not see Q2 on page 2.

The rough of the script placed at the TOP of page 2 would contain the following:

%%responses = sgapiGetValue(xxx);
%%count = sgapiCount(%%responses);

if (%%count <= 3){
  sgapiJumpToPage(yyy);
}

xxx represents the question id of the checkbox question (Q1). You can click the Advanced Edit button (paper and pencil) to edit a question and the right side has the question id.

yyy represents the page id you are jumping to. This is a bit trickier. Hover your mouse over the 'copy page' or 'delete page' links on the page you wish to jump to. Look at the URL in the status bar of the browser (bottom) and near the end it will have something similar to: qid=4. In the example, 4 would be put in place of yyy.

–

There is a function to hide the question rather than page jumping, but this was the simplest option.

Update 10-29-08

If you have 'other' boxes in the checkbox question, there is a need to update the script as follows:

%%responses = sgapiGetValue(xxx);
%%count = sgapiCount(%%responses);
foreach (%%responses as %%value){
    if (%%value == ''){
        %%count = %%count – 1;
    }
}
if (%%count <= 3){
  sgapiJumpToPage(yyy);
}