Another solution is to make every cookie have its own name. That is, rather than having cookies named search1=date, search2=carrot etc., create cookies that are named eggplant=true, fig=true, date=true, and so on. That way, you can set multiple cookies and you do not have to worry about any conflicts.

At this point, you would probably set the expiration date for each cookie to 1 week into the future, and create an all searches in the last week section instead of a last three searches section.