[EpiData-list] Select top 3 among 8

Hello all. I have a small constraint in my chk file and I wonder how I could program it. I have 8 options (8 variables V1 - V8). Interviewers are asked to chose their top 3 options. So that the legal values are 1, 2, 3 and there must not be the same value twice. I dont know how to write short instructions to check that. Thanks for your advices.

Serge.