Asus Radeon 6570 low-profile card - this is a descent card, you won't be running the latest games on max graphics, but it can handle low-medium range. I'm not so sure how you'll do with a 3-monitor setup however..
EDIT: If you can get the low-profile 7750 for under $150, then definetely go with that. That is an excellent card, and will run multiple monitors
Any advisable workstation GPU? or would it be out of budget?
You should be able to get Quadro 400/600 or Firepro V3900 ~$150, but those 2 only have 2 Video Outputs.
Other solution is, buy Quadro 400/600 or Firepro V3900 to drive you 2 displays, and use Intel HD4000 to drive the 3rd display.
EDIT : Make sure those setup will work out as you wanted them to, personally I never tried running workstation GPU and Intel HD4000 at the same time. I know it will work on non workstation GPU and Intel HD4000.