Hi Chris,
If by “ping” you mean email, one crude but effective(ish) approach would be to train User 2 to Assign the Request to User 3 when the time comes, and for User 3 to have their alerts set to email them when they are assigned to a Request. From there, User 3?can then finish off and send the @mentions as you’ve described, or (bonus) if there is a problem, Assign the Request back to User 2 for correction.
On the other hand, if (rather than email) you can train your team to monitor their work from within Workfront, I invite you to consider these One chart to Rule Them All and/or Switchback Workflow posts for inspiration.
Regards,
Doug