I am curious to pick out Flock House virus particles from my micrographs. Based on published information, FHV capside is under T=3 Icosahedral symmetry. In refinement step, what parameter of symmetry should I use? I tried T3 and I3, both gave me an error but only I1 worked, it seems a correct output, but does I1 mean just Icosahedral ?

The number that you specified (T=3) is just the T number defined by the h and k values of an icosahedral virus. This link explains the T-numbers well (Icosahedric capsids, Caspar and Klug ~ ViralZone). This means there is no T3 or I3 point group to apply as I in itself is a group of symmetries and doesn’t need a numeric qualifier. “I” should be good enough for the refinements to work.