Theme: The Three Amigos - synonyms.
20A. Seaside shade provider: BEACH UMBRELLA
37A. Philatelist's volumes: STAMP ALBUMS
44A. Practice that develops a variety of employee skills: JOB ROTATION
59A. Kid's imaginary companions, and what's literally found in this puzzle's circles: SECRET FRIENDS
Argyle here. Bit more secret with out the circles but you should know where to look for them, spanning the two words. Definitely a male slant to the entries but are there feminine equivalents?
Across:
1. The "m" in E = mc²: MASS. This formula states that the equivalent energy (E) can be calculated as the mass (m) multiplied by the speed of light (c) squared.
5. Put into force: ENACT
10. Wood-shaping tool: ADZE
14. Severely damaged sea: ARAL
15. Morning show featuring Al Roker: TODAY
16. Drubbed or whipped: BEAT
17. Calf-length skirt: MIDI
18. Lady's canine love: TRAMP. "Lady and the Tramp" Disney Movie (1955)
19. Omar of "The Mod Squad": EPPS. Popular guy in crossword land.
23. Bone: Pref.: OSTEO
24. Second person: YOU. I am first person.
25. Grammy category: RAP
28. "Don't play" music symbols: RESTS
32. "I'll do that!": "LET ME!"
34. Opposite of pos.: NEG.
40. Soothing succulent: ALOE
42. Inclined: ATILT. 'A' words. 53D. Come to light: ARISE
43. Sitter's nemesis: BRAT
47. Letters after ems: ENS. Alphabetically speaking.
48. "Not __!": "Quiet!": A PEEP
49. Macho guy: HEMAN
51. Not agin: FER
52. Musician's gift: EAR
55. Curriculum __: résumé: VITAE
64. Remove a rind from: PEEL
66. "Ask me if __": I CARE
67. Pigeon banter: COOS
68. Drops the ball: ERRs
69. Country south of Egypt: SUDAN
70. French I verb: ÊTRE. (to be)
71. Path behind a ship: WAKE
72. Poet's muse: ERATO
73. Taken in, as a movie: SEEN
Down:
1. Cuban dance: MAMBO
2. Spring zodiac sign: ARIES
3. '70s-'80s Egyptian president Anwar: SADAT. Ended in assassination.
4. Deli machine: SLICER
5. Caesar's ides-of-March words: "ET TU?"
6. Good name for an average guy: NORM
7. Not much (of): A DAB
8. Top-selling Toyota: CAMRY
9. Blood bank spec: TYPE 'O'
10. Second son: ABEL
11. Act of leaving: DEPARTURE
12. Microwave: ZAP
13. Sci-fi beings: ETs
21. Long-handled garden tools: HOEs
22. Break in the action: LULL
26. Capital of Jordan: AMMAN
27. Annoying types: PESTS
29. "Now!" in the OR: "STAT!"
30. Brit's sign-off: TA-TA
31. Northampton women's college: SMITH. Smith College is in Northampton,MA. Established 1871 (opened 1875).
33. Flow back: EBB
34. City south of Baghdad: NAJAF. Minneapolis and Najaf are Sister Cities since 2009.
35. Wed on the wing: ELOPE
36. Lose it big-time: GO BERSERK
38. Ballet dip: PLIÉ
39. Tiny physics bit: ATOM
41. Poetic palindrome: ERE
45. Oil gp. that includes Iraq: OPEC. (Organization of the Petroleum Exporting Countries)
46. "Avatar" race: NA'VI. The movie.
50. Nephews' sisters: NIECEs
54. Happen multiple times: RECUR
56. Govt. security: T-NOTE
57. Love dearly: ADORE
58. German steel city: ESSEN. "European Green Capital 2017"
60. Beyond that: ELSE
61. "Look what I did!": "TA-DA!"
62. Group with pledges: FRAT. (college fraternity)
63. Nevada gambling city: RENO
64. Church bench: PEW
65. Notable time: ERA
Argyle
