The third tier of football in Germany is called 3. Liga (not Bundesliga 3), that's why you probably couldn't find their portal which is located on DFB's website, which shows that the matchday 26 game TSV 1860 München - Hansa Rostock will be played on 1st and 4th of March, 2019.
1860 München's website tells that this game will be played on 2nd of March, 2019.
Hansa Rostock's website shows this date between 1st and 4th of March.
I would take all of these dates with a grain of salt because even Bundesliga has scheduled the exact days only till its 6th matchday.
As I said, the official website of 3. Liga is the one on DFB's, so you should check it first. My observation is that as currently 2nd matchday is going on in 3. Liga and they have published exact dates and times till 10th matchday, then it's logical to assume that the exact dates and times of matchday 26 won't be published eralier than 18th matchday. So, I guess it will be published somewhere around 7th and 10th of December.