B31.3 have a different approach than B31.1 for example.
If you access the help for Eff in Caesar input spreadsheet for allowable stresses you will see the following explanation:

B31.3, B31.4, B31.4 Chapter XI, B31.8, B31.8 Chapter VIII, B31.9, NAVY 505, Z662 (J), BS 806 (e), CODETI (z), FDBR (vl), GPTC/Z380

Allowable stress or yield stress tables do not include longitudinal weld joint efficiencies. Eff is ignored for the flexibility calculations. SH is multiplied by Eff when calculating the minimum wall thickness.

Due to this particularity, it it not required to have different material entries for different classes, the allowable will be the same. The only thing is that you will have to consider Eff only for minimum wall calculation.