The pair are two of 30 entered in the event, which is highlighted by middle-distance star Nick Willis and defending champion Hamish Carson.
Willis, Carson and Eric Speakman, who is also entered in the race, all ran sub-4min miles at the Cooks Classic in Wanganui on Tuesday night.
Carson won the event in 7min 56.19sec last year, slower than his personal-best time of 7min 49.24sec, set in Germany in 2011.
Speakman has never broken the 8min barrier, but showed good form to stick with Willis and Carson over a mile in Wanganui.
Commonwealth Games 1500m runner Julian Matthews, Aucklander Malcolm Hicks and Australian Craig Huffer are also in the impressive field.
Hicks finished second in 7min 58.58sec a year ago and is expected to push for a podium spot tonight.
Baan (20) finished 13th in a personal-best 8min 30.02sec a year ago, and will be doing well to finish inside the top 10 tonight.
The Caversham runner won national titles in the junior men's 5000m and 8km two years ago, but is not a specialist 3000m runner.
Shields (27), of Hill City-University, will also find the going tough tonight. He ran the 3000m at the Lovelock Classic in Timaru in 8min 39.22sec a couple of weeks ago, and his personal best is 8min 37.27sec, set at the Otago championships last year.
Hanna English is the lone Otago runner in the women's national 3000m championship.
The 18-year-old Ariki athlete also faces a tall order, with 2012-13 national champion Camille Buscomb entered.
Angela Petty, who has already qualified for the 800m at August's Rio Olympics, is also entered in the 15-strong field.
The 3000m races are being held in conjunction with the annual Capital Classic.
A bunch of Otago athletes are in Wellington for the meet, including Otago 400m record-holder Andrew Whyte.
He is chasing competition in a bid to clock a B qualifier (45.80sec) for Rio, and is one of 20 runners entered in tonight's 400m race.
Other Otago athletes competing across a range of events in Wellington: Christina Ashton, Maddy Spence, Felix McDonald, Ellie Duncan, Terry Collins-Hawkins, Scott Murfitt, Danica Davies and Rory O'Neill.
● The Otago 3000m championship, which was slated to be held during the twilight meet at the Caledonian Ground on Wednesday night, will be held as part of the Otago track and field championships on February 20-21.
