Hello Bruce
First of all, I’m assuming your R2 is the R1 in the lesson, and R1 and R3 are ISP2 and ISP2 respectively.
The neighbor 192.168.12.1 filter-list 1 in command should only filter BGP routes that are being received from the neighbor on which they were configured, namely ISP1 (192.168.12.1). This command should not filter out the routes received from ISP2, so something else is taking place in your topology.
As for the deny 3$ statement, this would deny any AS that ends in 3, that is the AS of ISP2.
If you want to deny this you would have to apply it as neighbor 192.168.13.1 filter-list 1 in so that you can filter the advertisements coming from ISP2. I also suggest you use the ^3$ argument in order to match exactly that AS.
Try this out and let us know…
I hope this has been helpful!
Laz