Rees-Zammit joined the Kansas City Chiefs and then the Jacksonville Jaguars, but did not play a competitive game in the NFL.
“It feels amazing to be back in rugby,” added Rees-Zammit.
“I went out there and tried it out but I am happy to be back. It is great to see all the boys again and there is a new coaching staff which is great.
“All the boys are loving it and we are ready to rip in.
“This is the first time I have been on this field for 18 months and it feels weird. It is great to see so many fans support us because it means so much.”
Rees-Zammit, Dyer and Davies underwent a series of running drills behind the Principality Stadium posts, while the main squad held a full public session.
Montpellier lock Adam Beard was not involved in the Wales training event in Cardiff because he had to return to his French club due to World Rugby regulations.
Uncapped Leicester number eight Olly Cracknell was an observer, while Dragons hooker Elliot Dee was training with the squad, but is not currently part of the 39-man party.
