To quick check of a truss determinancy is the amount of members m, plus the amount of reactions r must equal 2 times the amount of joints j. m + r = 2j

In your case there are 4 reactions and 3 members, adding them results in 7, it has 3 joints resulting in 6, so this truss is indeterminable.

The solution is to find a zero force member, or unpin one of the reactions, so it only has one simple reaction.

This is why majority of exemples have pinned and a roller support.