Option 1. If you can print to index cards at all, this should work, and is the easiest to do out of the box:

1. Pick your calendars.
2. Click onto the "Week" tab to show the whole week.
3. Click the "Print" link.
4. Instead of actually printing, hit the "Save As" button to save a PDF.
5. Open the PDF, and print it to a 3x5 card. Make sure you scale the PDF down to fit the selected paper size.

The fonts aren't the best by any means, but this is the easy way.

Option 2. A more involved way, which may or may not even be feasible:

1. Install gcalcli and its dependencies (Python, Google Data libraries for Python, etc.).
2. Run gcalcli to print this week's calendar: ./gcalcli --user USERNAME --pw PASSWORD calw 1

+----------+----------+----------+----------+----------+----------+----------+
|Sunday    |Monday    |Tuesday   |Wednesday |Thursday  |Friday    |Saturday  |
+----------+----------+----------+----------+----------+----------+----------+
|14 Oct ***|15 Oct    |16 Oct    |17 Oct    |18 Oct    |19 Oct    |20 Oct    |
|          |          |          |          |          |          |          |
|          |          |6:00am    |          |3:00pm ME |          |          |
|          |          |Planned   |          |4020 Lab  |          |          |
|          |          |Power     |          |          |          |          |
|          |          |Outage    |          |          |          |          |
|          |          |          |          |          |          |          |
|          |          |9:00am DOE|          |          |          |          |
|          |          |Meeting   |          |          |          |          |
+----------+----------+----------+----------+----------+----------+----------+

Or run gcalcli to print this week's agenda: ./gcalcli --user USERNAME --pw PASSWORD agenda 20071014 20071020

Tue Oct 16   6:00am  Planned Power Outage
             9:00am  DOE Meeting


Thu Oct 18   3:00pm  ME 4020 Lab

Print the text via whatever method you'd normally use.