I'm not sure that I am the correct person to answer the question of whether to sell individually or as a complete box. I don't have enough "selling " experience. I am mainly a collector that doesn't sell as much as the average chipper.
I have had full boxes of chips in the past and tried to sell as many singles as possible...but only sold 7-8 chips.... Another time...20 chips.....another time 12 chips.
Hopefully others might reply with advice as well.

If you are not a collector that is trying to save a couple for your self (in my opinion) I would sell them in boxes. There are probably 12-15 serious California collectors that would buy a set of the 3 if they don't already have any of them. I figure you could sell 20 sets+\- and might get stuck sitting on the rest. If you offer them "cheap" enough (3 for $10) I think you could sell some to people who might not normally buy "California" chips. ****All these numbers could be higher or lower****

Hypothetically ....if you sold 12 sets(3 chips...1 of each color) for $10-15 a set. You would net $120-180. If you sold the 3 complete boxes you might end up with $50-75 per box ($150-225)
It really just comes down to if you are in a hurry to get rid of them or you want to be a seller of chips for the next year.

We need to try and research the Rex and confirm an address. That would help with values.
What geographic area did you find them? California, Nevada, Florida.....? I'm just curious of how far they might of traveled from Taft, Ca 😀 That info might also help if we find and owner of the "Rex" from Taft and find he moved to Florida or where ever the chips were found. Thanks for sharing the chips with this Community. I do wish you the best of luck on whatever you plan to do.