In the upper left is says 二上り = niagari, which in the key of C is C-G-C.
Shamisen don’t have set tunings, so technically you can start with any note as long as you maintain the distance between the tones. The second chart under section 3 of this site shows the different possible tunings: https://www.shamisen.ne.jp/shamisen_life/shop/report8_tyougen.html
Kyle provides a wonderful explanation in this thread: