The amount of threads it uses, is not equal to the amount of simultaneous deployments.
With your settings it will use up to max 3 threads to process the started deployments.
The more threads you have, the faster it will go.
For example: lets say, for the sake of argument, starting a deployment takes 1 second.
You configured Lansweeper to only use 3 threads.Deploying on a group with 3 assets will take 1 second.
(3 assets, 3 threads => each thread runs 1 time)
Deploying on a group of 30 assets will take 10 seconds
(30 assets, 3 threads => each thread runs 10 times)
You configured Lansweeper to use 15 threads.Deploying on a group with 3 assets will still take 1 second.
(3 assets, 15 threads => only 3 threads run 1 time)
Deploying on a group of 30 assets will take 2 seconds
(30 assets, 15 threads => each thread runs 2 times)
Hope this answers your question.