There are three pipelines in my drone.yml file, is it possible to ignore one of them (for example, a third one) and launch it only by pressing the deploy button in the web ui?
Yes, you can conditionally limit pipeline execution by event type. Below are some links to documentation that demonstrate how to conditionally limit pipelines, and how to conditionally limit pipelines to promotion events.
https://docs.drone.io/pipeline/docker/syntax/trigger/
https://docs.drone.io/promote/