EDIT to add:
Mavs receive:
Grant Williams
Two future second-round picks (via Celtics)
Celtics receive:
2024 second-round pick (via Mavs)
2025 second-round pick swap (via Mavs)
2028 second-round pick (via Mavs)
Spurs receive:
Reggie Bullock (via Mavs)
Unprotected 2030 first-round pick swap (via Mavs)
https://www.nba.com/news/grant-williams-three-team-trade
(Thanks to zshawn10 from the GB for the info above)
EDIT to add:
Welcome, Reggie!