Why is this 240V / 20A motor labelled 3HP?
The current marked on a motor should be the continuous, full-load operating current. The power marked on a motor should be the full-load mechanical power delivered by the shaft. A 3 Hp motor should deliver about 2240 watts. 2240/4600 is 0.487. That tells you that the motor's full-load power factor multiplied by  its full load efficiency is 0.487. For a rough estimate, you could say that full load efficiency = full load power factor = square root of 0.487 = 0.7. That is not very good, but single phase motors generally have a lower efficiency than a comparable 3-phase motor. The power factor can vary quite a bit.
Since this motor was likely made more than 55 years ago (postal zone rather than zip code) and is a design type (repulsion-start, induction run) that was abandoned long ago, you can pretty safely assume that it is not very good in terms of efficiency and power factor.
More About Interpreting Motor Nameplates
There are online resources that explain in detail how to interpret the nameplates (rating plates) of motors that have been manufactured to international standards. Note that rating plates or labels that have less information than the one pictured in the question likely do not conform with international standards. Interpret those with great caution.
More About NEMA
NEMA is the National Electrical Manufacturers Association of the United States. It is a standards developing organization led by people associated with manufacturing companies. NEMA is accredited by ANSI, the American National Standards Institute. ANSI is a private, not-for-profit organization dedicated to supporting the U.S. voluntary standards and conformity assessment system. ANSI is the official U.S. representative to the International Organization for Standardization (ISO) and, via the U.S. National Committee, the International Electrotechnical Commission (IEC).
Motors and generators that have a rating plate like the one pictured in the question are generally assumed to conform with the applicable NEMA Motors and Generators standard, such as ANSI/NEMA MG 1-2016. Data marked on the rating plate should be interpreted with reference to that document. There is an ongoing effort to “harmonize” NEMA MG 1 with the similar IEC standards. The basic meanings of the data items have been pretty well harmonized. Items like “CODE F” and “DESIGN M” must be interpreted by referring to tables published in the standard. It may be possible to find important MG1 information and tables quoted or reprinted in documents that are freely available online.