Alison,
First, I think you'll want to define a "session." This will be variable "x." Variable "y" will be the increase in lead score you want to award someone who's had 3 sessions this week.
Then, you can create two separate campaigns and one field to achieve this:
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Create a new score field called "sessions this week"
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Create a smart campaign that runs every x amount of hours. In the smartlist, you should have "visited web page" in the past x hours. In the flow, add 1 to the "sessions this week" score.
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Make a smart campaign that runs at the end of every week. Smartlist should be anyone who has sessions this week > 0. Flow should be increase lead score by y amount IF x >= 3. Then, make a second flow step to reset "sessions this week" to 0.