Bought three cans in February, two at one time and one the next day.  Received shipping info last week on one of the two I purchased on the same day and the one I bought the next day, nada on the other one.  I know they don't go in chronological order, but I assumed that they would come in within a week of each other.  What's normal?

I don't think there is a "normal". I know I had two pending with SS one time that the F3s were approved within a day or so of each other, but I didn't get the shipment email on the second for a couple weeks due to a backup in shipping. They don't send an email for the F3 approval, just the shipment notification.

You could either call them, send them an email, or an IM...with the SN and they'll let you know.

I followed your advice BigWaylon and called them.  The form 3 is not back yet and should be, so they are going to follow up with the ATF.  I am very pleased at how helpful the folks at Silencer Shop have been so far.