For this achievement you have to buff the health of 3 team mates at the same time using a Field Regen Unit.

This will require you to be Level 20 or Rank 5 and to have purchased the Field Regen Unit Ability.

Once you have done that, start up campaign Day 1 and change to a medic at the command post. Then walk back to your spawn and put down the Field Regen Unit. Now simply sit back and wait for your team to run in, die, and then respawn into your Field Regen.