What size replacement fuse is required for this hot tub?
A quick google search lead me to this datasheet: http://www.cooperindustries.com/content/dam/public/bussmann/Electrical/Resources/product-datasheets-a/Bus_Ele_DS_2017_GMA_GMA-V_Series.pdf

There is one part (GMA-3.15-R) that matches your description and the current rating is indeed 3.15A.

If you're wondering why the current rating is such a weird number, it's because it is a preferred number (Renard series):


  R10: 1.00  1.25  1.60  2.00  2.50  3.15  4.00  5.00  6.30  8.00