Buff: Watch a LIRR AM or PM Peak weekday period at Forest Hills or Kew Gardens Station.
Either station would be good and have ample outside access and amenities nearby.
If you plan to ride Jamaica would be the place to keep watch for M3 MU trains in service.
As you likely know the difference between M3 and M7 MU cars spotting them should be easy.
Be prepared to ride to Mineola - another good place to watch LIRR trains - and buy your ticket(s)
before boarding to avoid any on-board penalty charges.
M3 cars are assigned to LIRR MU trains at random - it is the "luck of the draw" on that given day.
I will second the thought about not needing to rush to ride and watch LIRR M3 cars at this time.
The new M9 cars are just beginning delivery - there would need to be enough M9 cars to replace
about 150 M3 cars. With MNCR choosing to rehabilitate their M3 fleet there will be a presence
of M3 cars in the New York Region for the foreseeable future.
See this related topic about M3 retirement:
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MACTRAXX
EXPRESS TRAIN TO NEW YORK PENN STATION-NO JAMAICA ON THIS TRAIN-PLEASE STAND CLEAR OF THE CLOSING TRAIN DOORS