Donaldson, 34, had 37 home runs, 94 RBI and a .900 OPS in 155 games for the 97-65 Atlanta Braves last season. His efforts helped Atlanta win its second straight NL East title.
The ex-Oakland Athletic, Toronto Blue Jay and Cleveland Indian has made three All-Star Games in eight MLB seasons and won the 2015 American League MVP.
Donaldson is the best free-agent third baseman left on the open market. Per Jason Catania and Thomas Harrigan of MLB.com, he ranked second in WAR (wins above replacement) behind only Rendon on the list of free-agent third basemen at the beginning of hot stove season
As Heyman noted, though, the Nationals have competition for Donaldson's services in the form of the Texas Rangers, Los Angeles Dodgers, Minnesota Twins and Braves.
Washington could also look to deal for Chicago Cubs third baseman Kris Bryant. Jon Paul Morosi of MLB.com reported on Tuesday that the Nats made an "initial inquiry" to the North Siders about a potential deal.
But Heyman said the "belief" is the Cubs would want outfielder Victor Robles in return. That may be a hard sell for the Nats, as Robles is a talented 22-year-old center fielder coming off a season where he smacked 17 homers and swiped 28 bases.
For now, Donaldson appears to be the Nats' best bet to fill the hot corner as Washington looks to make a return trip to the World Series.