Donaldson will be 33 when 2019 rolls around. As well, he's in a FA class that includes both Harper and Machado. I don't think any team will offer him a 7-8 year deal as a FA based on his age and the current offseason trends we've seen when it comes to aging players/sluggers. I think he'll easily get a 5 year deal + a club option perhaps. 6 years might be the tipping point in getting him locked up.
I'll do a rough breakdown of every team and their 3B situation come 2019:
AL East: Boston has Devers, so that eliminates one big payroll team. Yankees likely will throw all there money at Harper & Machado, so that would take them out of the equation. If they miss out on Machado, then JD is definitely in play for them. Tampa and Baltimore don't have the money.
AL Central: Cleveland has Jose Ramirez on a very team friendly deal. Detroit is likely in full rebuild mode by then, Chicago nah, Kansas City doesn't have the money & the Twins aren't really a fit.
AL West: Mariners have Seager still locked up, Astros have Bregman, Angels likely will try to save money & resign Mike Trout or trade him before 2020, no to the A's. Rangers could be a possibility especially if Beltre retires after 2018.
NL East: Nats still have Rendon, doubt the Marlins have the $ to sign JD, and the Phillies have Franco. Maybe the Mets or the Braves could have interest & have a need at 3B. Mets have the money obviously, and if the Braves have a solid showing in 2018 with all their young talent, they might be looking to add a major piece for 2019. As well, Atlanta would be close to Donaldson's offseason home in Mobile, Alabama.
NL Central: Cubs have Bryant, Brewers & Pirates don't have the $ and the Reds have Senzel. The Cardinals likely could be in play for JD.
NL West: D'Backs have Lamb, Rockies have Arenado, and the Padres don't have the $. The Giants have a big hole in their lineup and at 3B (unless they think Christian Arroyo is the answer). Plus, Donaldson has experience living in the Bay Area when he played with Oakland. The Dodgers have unlimited resources, though they already have a good guy at the hot corner locked up until 2020 in Justin Turner. Still they could be creative if they want and move Turner around to a different position if need be.
So the teams I have listed who could be interested and have a need at 3B when 2019 rolls around are the Yankees (if they can't sign Machado), Rangers, Mets, Braves, Cardinals, Giants & we'll throw the Dodgers in there because of their unlimited $.