Well the restriction you mention only applies if you have been granted "subsidiären Schutz" (subsidiary protection) and after 16 March 2016. (In this case your relatives would not be allowed to apply for Family reunion until 16 March 2018). But if you have been granted normal refugee protection (which I think is the case as you have been granted permission to stay for 3 years not only 1 year) this restriction does not apply for you.
Procedure of several months is normal for applying for a visa for Family reunion. The next step will be that your relatives should receive a date for an appointment at the German embassy/consulate in Bangladesh . The German embassy/consulate is in charge. Any questions regarding the time-frame of the application should be adressed to embassy/consulate in Bangladesh.
However, your relatives will also be asked to provide documents of you (in copy).
The Ausländerbehörde here in charge of you will be in contact with the embassy / consulate in Bangladesh but is not allowed to communicate with you directly.
Once your relatives have received a visa from the German embassy/consulate in Bangladesh they must apply for a permission to stay at your Ausländerbehörde within the time-frame that the visa is valid for.
Pls feel free to ask if anything is unclear.