I can't think of a way to do this with standard configuration. You could try using universal persistence to track the client and corresponding SNAT IP. Else, you could try doing a calculation against the client IP to select 1/3 of clients use SNAT address 1 and not track which clients use which SNAT IP. The logic could be something like "if the last digit in the IP is 0-3 use IP 1, 4-6 use IP 2, 7-9 use IP 3. Obviously this wouldn't lead to a perfect distribution, but it would make for a low overhead way of using the same SNAT IP for the same client IP over any time period.
Aaron