Is there a protective cover available for using PS-3S Electrode Holder outdoors?

Use F03-11 Protective Covers.

Mounting F03-11 Protective Covers (Sold Separately)

The Protective Cover can be mounted only by using F03-12 Frames (sold separately) as a flange, or by using F03-12 and F03-13 Frames (both sold separately) together and embedding them in a concrete tank.

Using a Protective Covers for PS-[]S (R)-series Electrode Holders

Attach F03-12 Frames to the bottom of PS-series Electrode Holders. (See figure below.)
Next, place F03-11 Protective Covers on top of the Electrode Holders and press on it until it clicks into place.

Note: The cap screw attached to the Protective Covers are not required for mounting.

Using Protective Covers for BF-series Electrode Holders (Applicable to BF-3(R), -4(R), and -5(R))

Remove the two mounting screws (M5 x 25) of BF-series Electrode Holders and attach the two cap screws (M5 x 25) provided with F03-11 Protective Covers.
Next, put Protective Covers over the top of BF-series Electrode Holders, and then tighten the two enclosed screws (M3 x 20 with washers). See figure below.

Note: Protective Covers cannot be mounted on BF-1.