Discovered 10/2 wired 3-prong receptacle for dryer. Is it ok to leave as is?
Replace it with a homerun of 10/3 and a NEMA 14-30.  It's the only way to be sure.

The original installer was a massive cheapskate with no understanding of the NEC -- even if the install was old enough to be grandfathered in under NEC 250.140 and friends, it still was a violation at the time of install as using an uninsulated wire for the neutral (grounded conductor) is only allowed when using type SE cable, not type NM as the original installer did:


  50.140 Frames of Ranges and Clothes Dryers. Frames
  of electric ranges, wall-mounted ovens, counter-mounted
  cooking units, clothes dryers, and outlet or junction boxes
  that are part of the circuit for these appliances shall be
  connected to the equipment grounding conductor in the
  manner specified by 250.134 or 250.138.
  
  Exception: For existing branch-circuit installations only, where an equipment grounding conductor is not present in
  the outlet or junction box, the frames of electric ranges,
  wall-mounted ovens, counter-mounted cooking units,
  clothes dryers, and outlet or junction boxes that are part of
  the circuit for these appliances shall be permitted to be
  connected to the grounded circuit conductor if all the following conditions are met.
  
  (1) The supply circuit is 120/240-volt, single-phase, 3-wire;
  or 208Y/120-volt derived from a 3-phase, 4-wire, wye-connected system.
  
  (2) The grounded conductor is not smaller than 10 AWG
  copper or 8 AWG aluminum.
  
  (3) The grounded conductor is insulated, or the grounded
  conductor is uninsulated and part of a Type SE service-entrance cable and the branch circuit originates at the
  service equipment.
  
  (4) Grounding contacts of receptacles furnished as part of
  the equipment are bonded to the equipment.


So, replace this abomination with a fresh homerun of 10/3 and a NEMA 14-30 receptacle -- you'll have to change the power cord on your dryer to a four-prong one, but that's a small price to pay for never having to worry about getting bit by your dryer again.