I am looking for Nigeria administrative boundaries (0-3) level especially the 3 rd level which is called "senatorial districts".
Where can I find these?
The GADM is a good resource for country admin boundaries, both internal and external.
Try these shapefiles: gadm36_NGA_shp
There are three levels available:
From your question you're asking for senatorial districts, of which there are 109. Each state is split into 3 districts and the composition appears to be based on local authority bounds (level 2). You could therefore try using the level 2 local authority bounds with the district composition to merge into senatorial district bounds.