This is pretty simple to do using the Job Group imposition (part of the output management license)
Create a 2 stage imposition tempalte.
Impose to a semi-automatic size on stage 1 and repeat the same document 7 times
On the second stage impose 3up cut & stack with the option to place jobs in separate columns
Finally, enable the Collect option on the imposition node in the workflow
I've enclosed the imposition template.
In order for the above to work, you need to ensure the three jobs are recevied by imposition as a Job Group.
You specify that during submission. However, in a lot of cases that's not practical. Insead, you can use a Collect node before imposition to aggregate the business cards:
However, you will then want to ensure there's a "plan b" in case you do not get three business cards in a reasonable amount of time.
As such, the collect node (also part of the output management license) has the option to release the job after a certain amount of time.
After that you will want to send jobs to one imposition or the other based on why they were released.
This is what that workflow would look like
I've enclosed the workflow as an attachment.